How to tell my gf to stop talking to this guy nicly?

ok i get realy jealous when he wants to talk to her thinking they are bffs(he told her that) ...she loves me alot and she tells me she doesnt really care about his friendship but still its not nice to diss him off(question 1: do u think she is right)......ques 2:how to tell her to cool it down with himm and to make him just another friend because im feeling insulted (ps: i could beat the hell out of him but then she will hate me then)...........answer ques 1 and 2 plzz