So I've concrete evidence he cheated on her with crack whores, we live in a city where prostitution is illegal so this is pretty serious. He did the same thing to me. After getting tested for stds, I tested positive for 2 stds. I believe its my place to let the poor girl know who was with him right before me what he did: he cheated on her with prostitutes. I know for a fact she doesn't know anything about this.. The worst part is that I'm sure she has all these diseases and doesn't know about it. I don't want to seem like a psycho but its for her own good. I can't live knowing someone has stds and not warning her to get treated. Help!

This is a case where you SHOULD contact her--her life may be at risk, and her fertility certainly will be if she is not treated.

If you know how to contact her, at least urge her to get tested for STDs.