You could try using one of those dictaphone things to tape anything this guy says,as it's obvious your b/f is so smitten with him,she won't listen to anybody's words or advice.This guy is obviously way creepy, by trying to hit on his girlfriend's bff.The only way to catch him out is to get the proof to convince her this guy is a rat and a cheat.
Best friends bf tells me that he's in love with me and tried to get me to makeout with him. Decided to tell her but she won't believe me and he won't admit it infront of her! Its getting too creepy!You can ask for more details?
Ok this is very bad. Hang in there. What you need to do is be there for your best friend. Tell her all you know and tell her you care about her feelings and its her choice if she wants to believe you. If she does, then tell her you think they shoud break up, but make sure you don't tell her that just so you and her bf can be together. Stay away from him because if he does it to one girl, he will do it with another. If she doesnt believe you then tell her thats fine and your going to be there for her when she gets hurt. Hope this gets a helpful! :)