Please let her know ASAP  if it me  I would want to know ... at least she  can then make the call as  to sticking it out  or moving.....if it was myself I would move  not only are you putting yourself  at risk  but more importantly  you are putting your client at risk if I was  a hobbiest and knew everyone who lived there was aware of her activities I would not want to be the showing up at  the wrong time at the wrong place....... I live in a home in AZ very quite area for 20+ yrs and .still some guys always ask  "do  you have any problems with the neighbors? " Men are ALWAYS concerned  about their safty first.....

You'll be doing her and your fellow hobbyists a favor by letting her know

If she's smart, she's figured out how to take care of the manager.  It's the neighbors that are usually the problem.

Every situation is different, but I knew a lady that worked out of the ground floor of a suite hotel for over two years.  She'd give the security guards "tips" and buy them little things- maybe a cup of coffee or some donuts.  She never had sex with them, though.

She never had to tell them what she was doing, and I don't think it's ever smart to do that.  She'd say that she had a lot of company and apologized in advance if they didn't park where they should, or something generic like that.  But I'm sure they could tell what was going on. She just  made it so they didn't make an issue of it.

Annoyed, or nosey neighbors are the bigger problem, so if they're talking about her, it's only a matter of time until one of them says something.  So yeah, I'd give her a heads up before it blows up.

geez what is this? maybe my spideys are bit sensitive but this seems fishy... if you are legit then call the booker and voice your concerns -  but be sure to mention that most of those who live around her don't seem to mind but do inform the new residents of her activities and even the manager knows... maybe the manager moonlights as the night time booker?

just giving a totally opposite perspective - which would've never happened if OP posted without alias... .o2

Give them the opportunity to make changes or to choose not to ( bad idea).  

I knew a provider in the West LA area who got arrested because of overly mosey neighbors who complained to the police enough.  She was from another country and sadly had to leave the country.  I think they might have even impounded the cash she had.  Terrible.

Uhm, seriously? You said she's been working out of there for a year and has a constant stream of men going in and out... Trust me, she cannot be foolish enough to think that people don't know what she does!
Put yourself in the neighbor's shoes! Single woman living alone with guys visiting...? It would take you 2 days to figure it out! So no, you don't need to "alert" her about that...
As far as the manager goes, it's a tricky situation. They tolerate her activity since she pays the rent, but if she were to complain about the mgr "spreading the news", they might retaliate.
There's nothing she can do about the neighbors, except try to stay on good terms with them and hope they won't be malicious if she is nice to them.
If you feel that you must do something, call the booker and tell her about the manager's gossip... But there is not much she can do. It sounds like a risk (and a huge one at that) they chose to take by having her work out of her apartment.

I've had a provider neighbor in the past and one of the nosy neighbors enlightened me almost right upon move-in. I just said reeeeeally and changed the subject. Little did she know she was talking to a "colleague" of the girl.
Then she explained to me how she knew. Supposedly she overheard some phone calls where the girl detailed her measurements and looks (in my mind, she could be a model!) and that she was often picked up in a car (could be someone who doesn't know how to drive!)... But then there were a few guys who came to visit her which really let the cat out of the bag.