JUST DO IT!!!!! If I were in your shoes and she is saying I would only blow my bf, but I would blow you too, get the bj first and see where it leads. If you end up slammin it so be it. She wants it, me hopes you want it. Get some Tiger.
Confront her on it and ask her if she was serious.. then tell her that's fucked up if she was. Then confront your friend about it.
I doubt that just not talking to her anymore would be too kosher. After all she is his homies girlfriend.. He'll have to be around her and talk to her at some point, and his boy would probably sense some awkwardness between them. OP- I would say just don't get with her unless you're cool with loseing a friend over a chance to get ur dick wet.
Tell your friend, and if he acts all Hollywood "your jealous, etc", say "Ok, watch this", and fuck her. And get proof. And bring the proof here. Then get them both out of your life.