My bff who's a boy is a couple with my bff who's a girl. The girl has said that she wants to cheat on her bf, my bff, with me. I haven't said yes because I don't want to hurt my boy bff, but I can't tell because then I'd lose my other bff?

You tell the girl "NO!" and tell her she needs to straighten up. I wouldn't mention it to guy, because often times that comes back at you. I would actually distance myself from the girl. I wouldn't spend any time alone with her. You should actually rethink that friendship. If she is willing to cheat on her boyfriend, and wants to put you in that situation, what kind of friend is she really? Not one I would want.