It's going to happen to you. Trust me. It may be her choice or yours, but ultimately it will happen. Here's my story:  
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We've been in a arrangement for 3-weeks short of 2 years. She calls me her BF and has me call her my GF when with my friends. Her friends for the most part don't know I exist - though they suspect she has "someone" in her life who's been giving her sugar/support/mentoring/etc. Her mother has met me and we've carefully skirted around any explicit discussion of my relationship with her daughter. But Mom has SD's of her own, she's not stupid. Only one of my friends knows I sugar and bang multiple chicks a month. He's jealous as hell and keeps my secrets.  
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We went to monthly allowance after 9 months together and it (unexpectedly) worked out well. She delivered on her side with regular time, sex, and travel companionship as agreed and asked. I delivered on regular allowance (about 75% of her rent), plus other small gifts on occasion like grocery shopping, birthday presents, paid her traffic fine, etc.  My total monthly investment added up to about $1,500 a month, which got me laid weekly or more, mostly on demand. Plus she happily did my laundry when at my place - something I actually valued almost as much as the sex because I hate laundry. LOL  
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I've been trying to coach her out of her poor financial state be helping her with career guidance and goal setting/attainment, as well as lots of discussions about how/why to truly prioritize her spending on what she NEEDS and not what she WANTS. That's been the hardest lesson for her to learn, and I'd say she's not making nearly enough progress to show that she's maturing from wannabe princess to responsible adult.  
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I know she has been seeing other SD's, and I've been ok with that. I never signed up to fully fund her monetary asks and I'd be a hypocrite to be bothered by her getting cash for sex for "needed" funds her job and I don't provide. As do I, she's very careful about positing references or pics of her time with a SD on social media. Careful, but not perfect. I have seen her pics going to a Lakers game, sailing on a party boat around the Marina, and attending celebrity/rapper/ultra rich private parties that she should never be able to access without a sugar path.  A few times she's posted a pic of her with another guy (presumably a relative or incidental stranger), or posted a vid where you can hear an unknown guy in the background (sometimes it was me).  I never ask about these guys and tell her I don't want to know. Not interested in creating some jealousy drama. Her time away from me is her time. Her cash - whether from my sugar, some other guy's sugar, or actual payroll form her job - is hers. BUT...  
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But she crossed a line I didn't know I had until she crossed it. And it upset me enough to drop her.  I'm in the process of rebuilding my rotation anyway, so I just need to fill one more slot (so to speak .   Here's what she did:  
She is an aspiring chef and has been applying for culinary schools and trying to get enough financial support/sugar to pay the tuition - somewhere between $30k up to $200k depending on the school. I've helped her from time to time by buying her chef stuff, taking her to celebrity restaurants and helping her learn how to save and budget.  From time to time she's asked me for cash to fund a chef-type event like attending a food festival, a celeb chef meet & greet, etc. When she asked me to pay $600 to get her a 3-day trip to Vegas for an international food expo, I was happy to help.  The red flags started there. 1. I could not find any food expo occurring in Vegas at that time .  I've been to Vegas a BUNCH and I know how to find scheduled events and navigate booking air, hotel, etc. But I drew a blank - could not verify the event was real or where it was, so booking the best hotel was a challenge. 2. She had a hotel preference (expensive, of course) but it was far from where she said the expo was and I knew she'd have to pay for a ton of taxi fares if she wasn't at or next to the hotel. I was not going to fund an additional $200+ in taxi fares when she could stay local to the event. We settled on a closer hotel but I was surprised she was not worried about getting around without a car or Uber (she can't get an Uber acct - long story).  3. I booked the trip and on the appointed day off she went.  3. No food pics or snaps. She's ALWAYS taking pics of her restaurant food. Always. But at an international food expo, nothing, and no pics of her with celeb chefs.  I even asked her by text how it was going, who she had met and what foods she had tried. Her answers were vague. What I did see was pics of her at the Mirage pool, pool bar, pool waterfall, and the major clubs on the Strip - the kind of clubs I could never get into without a 20-something hottie or four by my side.  But that wasn't the crossed line. It was who she was with in the pics that clinched it. She was there, on my generosity, on the premise of helping her career, on extra sugar beyond her agreed-to allowance, to party with another guy.  A lot. I was able to back track him from his social media account - they were both posting and "liking" each other's pics and vids. And that's what pissed me off. Like I said, I don't care who else she bangs. I do care if she lies to me for money to bang someone else.  
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Conclusion: I didn't talk to her for a week after she got back.  Just left her on ignore. Then I told her I'd be stopping by her place and she replied yes - she was super horny.   Got there at the appointed time and just explained that I was done with the arrangement. I needed to move on and make some changes in my life, blah, blah, blah... She cried (because she realized she'd just lost about 75% of her income, and perhaps she'd lost a valued friend as well), and asked a few questions: was there someone else, am I mad, are we still friends, etc. I never busted her; didn't want the drama. I was not angry or emotional, just calm and rational. But I was prepared to bust her if she tried to push me for or negotiate for another month or two of rent. She didn't (to her credit). So now I've just reclaimed a HUGE budget for future sugar and, as I've said so many times before....
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Life is good.  
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The Cat

I'm glad you had a great time while it lasted.  Expectation is what screws up pretty all interpersonal relationships.  But the SB/SD relationship is based upon essentially a lie, e.g. bf/gf but the lie is you need to pay as you go for that pretend relationship.  It seems she either wanted to party alone and have you pay for it.  Or maybe the guy she went with just didn't have the cash to cover both of them.  Whatever, now her income is down 75% and SA's rules are now fucking awkward that so I, as are many SB's, are hitting me up from a year ago to see if I want to play!  So it's easy to stay busy and save SA fees!

I think you should have made it really crystal clear why you were dropping her.  I don't see that as drama, more like a continuing part of the education and mentoring you've been giving her all along.  It's not ok to treat people the way she treated you, and if you do, there are consequences.  I think you should have told her.  Anyway, lots more fishies in the sea and now you have more bait available to catch some!

I don't have SD/SB experience, but I think you should tell her why you're calling it quits. There may be an ever so slim chance that she has an explanation (and proof that she attended a real event). There may be an ever so slim chance that she was planning some sort of surprise for you (good surprise, not bad surprise) -- some special food thing she's learning to cook? -- after a few more days or weeks, now canceled. There is an ever so slim chance that the guy she was with was and social media-ing with was her cousin or BFF's husband or some other "dear friend, not what you think" kind of thing.  
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And, as sweetman pointed out, if she is guilty with NO excuses, she can learn from your disclosure instead of suffering over "What did I do? What did I do?"  
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Maybe you'll patch things up ("make up sex") or maybe you will move on. No matter what happens, good luck!

I have my eyes wide open where it comes to seeing both hookers and SB's, but there is one thing I absolutely, positively REFUCKINGFUSE to do, and that is to be the pathetic old man who is paying not just his SB, but her "real" loser boyfriend to boot. My ego simply won't allow it.

 
I remember several years back a friend of mine had an SD/SB relationship with a hooker, if he told me once he told me a thousand times what "chemistry" he had with her, so much so I honestly got sick of hearing about it. One day he told me he was going to be free that weekend to go have beers because his "girlfriend" had an emergency trip she had to make for her sick mother and that while he was going to end up giving her close to two grand to go see her mother without even getting any pussy, that she was going to make it up to him the next week.  

 
Well it so happened that his hooker/SB worked for an agency who's owner was a mutual friend of our and she confided in me that she didn't have a sick mother, she was going away for the weekend with her "real" boyfriend on my buddy's dime. We both agreed that while we felt bad for him neither of us could bring ourselves to tell him knowing that it's always the messenger who gets shot. Not to mention we both knew how smitten he was with her and neither of us wanted to be the one to burst his bubble and ruin his fantasy.

 
Since then I have made it a point to NEVER be "that guy" I don't care who my SB's fuck, and I don't expect them to be "faithful" to me as I am hardly being faithful to any of them, but I absolutely positively refuse to "pay" for her to be fucking some other guy.

 
I honestly don't know if I were in Herb shoes whether or not I would have said anything to her, but I am sure about one thing, there wouldn't be any make up sex, because whatever I would have had to say would most definitely be in GaGambler terms as in "fuck you, get the fuck out, and THIS is why". Knowing this about myself, I just don't see anything to be gained by telling her, and most likely I would just quit returning her calls until she got the hint.

 
BTW Imp, it is clear you don't have SD/SB experience, the chances of her having a legit excuse are ZERO. Herb is hardly a rookie at this, if he "knows" what she did, I am 100% confident he's right about her. Sometimes you just "know" these things.

Thanks for the perspective, GaGa.
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Some additional context for the peanut gallery:  
1. I have pics (screen shots on my phone) of her with the guy. There is no other way to interpret these shots: Her arms around him in the resort pool, his arms around her, a pic of them in evening wear at the floor-to-wall display sign for the event HE was working (he's an event planner/DJ/entertainment industry leech).  
2. Vids of her referencing how glad she is to see her "boo",  twerking in the pool while he smacks her ass, taking an Uber ride together on the Vegas strip.  
3. A snap relaying a conversation with said "boo": HIM: Boarding my flight I'm on my way back to you baby" HER" Can't wait to see you boo!"  I fly a lot, but I wasn't when this posted and she's never called me "boo".  
4. In the last 4 months, she's mentioned several times that she got invited to a number of "rich person" parties (Rappers, athletes, Beverly Hills crowd). At first she implied she was using a former-stripper-come-super-SB friend of hers. Indeed she was for some events. And I assume she was using those to suck some famous dick for cash. But her snap posts didn't line up with the friend's posts often enough. She had to have another way into these events. Hence the douche mentioned in #1.  
5. I was not, and am still not interested in "prosecuting" allegations with her. I saw no need to deal with more drama than just cutting her loose. No need to have to "prove" my findings or try to get her to admit it (Godfather style: I know it was you, Fredo…).  I just wanted to end it and move on. Also, although I value the opportunity to share and teach my fellow SD's here, I feel no such obligation to teach her how to better deceive a future SD.  
6. Last thought about how to part ways: We constantly repeat the NSA mantra. That means I have no need, obligation or reason to tell her anything besides "Bye, Felicia!"  Nor does she need to give me any explanations if she wants to bail. This has been proven many times by past SB's that simply ghosted on me, NCNS/Block/Ignore, etc.  
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I'm not losing any sleep over this. In fact I just lined up my 1st BCD with one of my new adds to my rotation:  24, spinner, sweet/demure disposition, and looking for minimal allowance. Bang time Thursday.  
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Life is good.
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The Cat

I would have to agree that you handled it the best way. She already knows what she did anyway....unless she's completely vapid. She's already paying the toughest price for her lack of discretion and class. Who knows how long it will take her to replace what she has lost....and it may come at more cost to her.Karma is a bitch! But you have definitely taught her a lesson in devaluing you in this case.