OP whatever you do, DO NOT tell your best friend you've known since grade school that his GF is cheating on him and is probably a prostitute and may have STD's. Listen, for all you know your friend is a psycho axe killer and will go crazy and kill her if you tell him. Plus, it may not even be true. Yes, she's has ads online where she is listed as a prostitute. Yes she has pictures of her up on those ads. Yes the phone number in the ad matches her phone number. But, you don't know for sure. Also, she may have already told him and he enjoys an open sex relationship between her and the strangers she sleeps with for money.
OP the best course of action is to do nothing. Don't rock the boat, just forget about it and go on as usual. However, if you must do something tell his GF first. Allow her ample time to concoct a believable lie and delete all evidence online before even beginning to think of telling your friend. But, whatever you do, and I repeat, DO NOT TELL YOUR BEST FRIEND FIRST.