let it go. you should never have to explain your relationship to someone. if you know what the deal is, and he knows what the deal is, then trying to explain it to someone else is pointless. and that's his girl. you can't convince her that it's all good. she has to realize that for herself.
theres nothing wrong with a girl being a man's best friend or a man being a girl's bestfriend. the only problem comes in when its the man's girlfriend or the girl's man. and the problem with that is the person doesn't trust the person their with subconciously. so i dont think theres anything wrong with it unless you dont trust your man or girl.