Awww, sorry to hear it az!

First things first, we dont really now if shes cheating on him. It is possable that az's gf is just good friends with his mate. But it is possable she is cheating on az. Whatever the case is az, try and find out now than play her game. If she is cheating on you, tell her its over and walk away first. It sure feels good to dump someone first when the other person has been cheating on you. I know cuz i've gone threw that twice. If your best mate is cheating with your girl its really shallow in my opinion. But hes ya bro and from one clown to another no girl can ruin that relationship. Good Luck az!