If she is a “former” prostitute, I would caution your friend about inherent psychological issues that she may have. From my understanding, prostitutes, as a survival technique, have to be able to compartmentalize their consciences (they become “someone else” when they are doing their business than what they are when they are with their friends and family). That would be a challenge that, at least as far as I can see, would cause some serious damage to the woman.
Obviously, there is the possibility of diseases that she may have gotten herself…some of which might not go away (e.g., Herpes, HPV, HIV). So physical, periodic checks for her (and if he’s already had sexual relations with her, for him as well) are an absolute necessity.
With those things, people can change, so if she is a “former” – I wouldn’t necessarily want to tattoo a Scarlet Letter on her, but there are cautions that need to be considered.
If she is currently an escort, I would caution to avoid developing any serious emotional connection. Not necessarily to cut himself off altogether, but to be very, very cautious with himself. That profession inherently involves a lot of good acting…telling the clients what they want to hear. So trust should, inherently, be part of the deal. Obviously, the above suggestions regarding a “former” prostitute apply in spades if she is a “current” prostitute.
Most important, in either case, people (in my experience, women more than men) get involved with other folks believing that they can reform the other person. That inevitably fails…people only reform themselves when they want to reform themselves.
As to whether it’s your business to counsel this friend? I don’t know how well you know this person and what your relationship is, so it’s sort of tough. But the above are things to think about.