Is there any polite way to inform a person of his girlfriend's awful reputation?

An ociate of mine recently began dating a woman who has a terrible reputation for being promiscuous and cheating on her ex boyfriends. He, on the other hand, has only been in a real relationship once before and is quite naive about the whole situation. I am pretty sure he has no idea of her background. He thinks he is in love, but I believe he is putting himself at risk by having unprotected with this woman. Should I warn him, perhaps even anonymously, or should I let him find out the hard way?