Why do you assume that there was something wrong about the OP being on BP? What if he's ok with admitting to his friend that pays for sex?Probably but isn't the OP a hypocrite for being on BP in the first place? Ye who is without sin cast the first stone.
I take your point about a material factor in this case being that the girl is a working girl. It is precisely because of that that privacy concerns are important....but not necessarily paramount IMO.
It wasn't my example that I hypothesized. It was yours. My point throughout this is that it's not what she does for a living that's at issue, it's the nature of the relationship itself.
If I were close enough to the guy to care AND be able to have such a frank discussion (and yes, have some idea as to how he would react), I wouldn't drop the bomb and walk away. Hell, if anything, I would probably be the best friend to have around for support.
But, perhaps we're going about this all ass backwards. Maybe the decent thing to do is to go to the girl and have a frank discussion with her. Maybe she's already told him. Maybe she doesn't want to tell him and has perfectly good reasons not to. And no, I don't mean to drop her a creepy FB message... We're all adults, and we can behave with courtesy and respect.