Given that she came onto him, and he denided her I'd say the OP is probably a pretty stand-up guy.
OP: tough call. Personally, I think love is more important than friendship (although both are very important). Basically, I'd talk to the guy-friend, but realize that that friendship might end if you start to see the girl. Also, consider that she is "on the rebound" and might be making rash decisions. One minute you're as close as can be, the next she's going to a movie with some other guy and hopes you can still be friends...
Actually, you might want to talk to her about it first. Tell her you like her and whatnot, but also that it's tough because you don't want to be hurting your guy friend. If she wants to pursue it anyway, then talk to the guy friend about it?
Eh, I don't know. Just do what your penis heart tells you to.