I would do this:
Sign up an anonymous fb acct or email acct or fongo acct.
Then send her a message that reads something like:
"Hi, I see you are X's gf now, congrats, he is a great guy and a good friend. I also know you are XYZ from backpage. Please don't worry, I am not going to out you. I am a friend, and positive towards women doing your job and the discretion that requires.
But I am also concerned for my friend. I expect that you will share your working status with him, so that he can make an informed decision about what he is getting into. If you do, you have all my support and best wishes. If you don't want to, and you break off with him, your secret is safe with me.
At this time I prefer to remain anonymous for my own reasons of discretion."
You might even link this thread, assuming your terb id is private enough as well. Note that you guys are all assuming the guy doesn't know. For all you know, maybe he met her through that bp ad and hit it off..