Have you ever had to deal with a SB who gained a lot of weight?  My current longest running arrangement is with a 25yo who was a slim cutie when I met her, but has gained a good 35lbs recently.  She's chubby now.  I probably would not have been interested in her had she been this curvy when we met.  BUT:  she's the most sexually open minded woman I've ever met, willing to do things no one else will do such as anal intercourse and rimming.  We have established a very affectionate friendship and have enjoyed lots of activities other than bedroom.  She loves to go riding on my motorcycles, and we've done other things as well.  So I'm not about to kick her to the curb for gaining weight.  And I still love our bedroom activities, even though she's not the visual treat she once was.  And my other 2 steady SBs are both slim works of art.  Just wondering if you have had similar situations and how you have dealt with them.

Wow, that's a lot of weight... not sure I could manage that but I haven't dealt with this situation (though it's pretty common for married folks).
Often that kind of gain in a short period of time could be a sign of a medical problem or that she's just bored.
I'd take her on a long hike or bike ride.. maybe offer to pay for gym membership.
I can't say I'm super fit but I think physical activity is important for both mental and physical health. She may be depressed (maybe not clinically but just down) and more actiity might help.

Not with SBs, but with wives.

1. Personal weight gain / loss. I gained around 50 lbs and 2+ inches around my waist during Covid. I went from a tightly cinched belt size X inches to the second hole on a belt size X+2 inches. Ugh. Fortunately, I was able to lose more than 60 lbs in ~90 days by just counting calories, drinking a lot more water than I used to and I needed no other tricks. No pills, no supplements, no Weight Watchers or Jenny Craig. I've gone from that X+2 inch belt down to an X-2 inch belt! I limit daily intake to ~1500-1600, sometimes only 1400. My weight is now holding steady. I don't have cravings or anything.  And, I feel great! I could probably peel off a few more pounds if I did 1300-1400 cals for a few weeks but I don't feel the need to ... unless I want to get back into my college job interview suit.  
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2. As far as the SB is concerned, I can only guess at some suggestions and these are NOT from experience. These two are touchy. You don't want to sound threatening or anything like that ("Oh no! He's going to dump me!"), but maybe bring up two *future* considerations:  
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(a) "We've got a great thing going here. But this might come to an end some day. ("Oh no! He's going to dump me!") If you could get back to your old self [weight], you would be able to find someone new in 5 seconds flat. In fact, my friend mrfisher told me about a related situation he was in. When you're back to your former self and turning heads, I just hope that YOU won't dump ME!"
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(b) "We've got a great thing going here. ("Oh no! He's going to dump me!") But I've noticed you've gained some weight. You're young and it might not seem like a big deal but it could lead to future health problems. You should develop good habits NOW so you can stay fit and trim and healthy for years to come.  My friend impposter told me that he lost a lot of weight (and 2+ inches from his waist) just by cutting back and counting calories. Let's try it together. The buddy system! Mutual support! I just hope that YOU won't dump ME because we no longer gulp down huge amounts to food together."  
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Damn. Now I'm getting hungry ... for an SB!

I'm impressed.  That is quite of bit of willpower and diligence to limit caloric intake to that level over a 3-month period.

I don't want to entirely hijack sweetman's SB post, but  ...  
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Having to buy the X+2 inch belt was a big hint, but when I looked down and couldn't see my dick without bending over, I knew I had a major problem. It was my New Year's Resolution and I started in Jan 2022. At first, I kept careful track of the calories (writing things down) and that made me very AWARE of what I was eating or about to eat. I was no longer just blindly opening a box or bag and eating until I was full. I was actually making decisions. Then, I just made what I thought were sensible tradeoffs:  
hamburger on a bun = 480 cals, hamburger without the bun = 260 calories. Should I skip the burger or the bun? I skip the bun.
1 oz potato chips = 150 cals, one 8 oz bag of chips = 1200 cals
I am honest enough to know that once I open the bag, I will not stop at one ounce ("Bet you can't eat just one [ounce]!") so I stopped buying chips altogether. Not a big deal, for me. I LIKE chips, but I don't miss them. (I've actually cheated twice since Jan but that just meant I ate ~1200 fewer "healthy" calories that day.)  
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I've replaced french fries with green vegetables at a fraction of the cals. I've replaced cakes and cookies ("Hmmm ... dooough-nuuuuuts.") with NO cakes and NO cookies. If I do have 2 Oreos = 140 cals, I have to skip a portion of baked chicken. Two Oreos = 140 cals?! WHO CAN STOP AFTER JUST 2 Oreos? Now that I'm keeping track, I can. And I know that the chicken is healthier for me than the Oreos so I rarely even buy cookies.
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The first 20 pounds came off FAST, less than 3 weeks. ("Wow! This cal counting is really working and is worth the effort!") That encouraged me to keep going. Weight loss slowed down, but the trend was consistent. I lost over 60 pounds after ~90 days. I think I've hit a fairly stable point (plus minus 3 or 4 pounds each day) or it's just coming off VERY slowly now.
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It's been 8 months and I'm still doing 1500-1600 cals per day and I don't have a problem sticking to that.  And I'm not a fanatic about it, either. Sometimes I have to guesstimate a cal count so I just try to be honest and not cheat or fool MYSELF. ("180? 200? 220? I'll just call it 200. Close enough.") Cal counting and cal restriction just turned out to be easy for me. I know it's not that easy for everyone ... or there wouldn't be WW, Jenny Craig, Noom, etc..

The fun fact is women get too comfortable in relationships.  I personally don’t feel sexy if I gain to much weigh. I have a home gym & stay active to be prepared for life. Buy her a treadmill, or a gym membership. Health=Wealth