Bros before hoes? idk help me :(?

Well I have this friend I met one night and he told me he was struggling with math. So i took him under my wing and tutored him (free of charge). And him and I started to hangout and became good friends. Well one night he tells me he is talking to this girl who happens to be one of my best friends that is a girl. Well this guy is somewhat of a loser. He smokes and drinks and has lots of sex and is only 13. Well my friend told me she really likes him and he told me he really likes her. But I feel like she can do A LOT BETTER and i don't like the idea of them dating. Should I tell her who he really is and ruin his chances with her? Him and I have been friends for about 2 months. Her and I about a year.