Mm, I think it was a relatively good choice. If she has a history of going after her friend's boyfriends, then she is obviously not trustworthy and not much of a friend.
I don't know. She only asked for advice, and she doesn't necessarily have to take your advice. And not being friends with her is a little harsh, but I would distance myself from her if you can't trust her. Not being friends with her is not going to keep her from stealing your boyfriend.
Also: If he really liked you then he would wait a while, and no guy is worth loosing friends over.
Well you just lost a friend over a guy.