Is this an acceptable friendship, or an emotional affair?

I honestly believe that the reason why he would have misrepresented his friendship with her to you is because of this reason right here. You are upset about it if he has known her for longer than you have been together than he really doesn't want to lose her friendship and if they were going to do something they had two whole years to do it in before you ever came along. You should really have more trust in your boyfriend. Now you need to ask yourself why you have such a problem with it, Is it insecurity or simple jealousy and deal with that issue. On a personal note i have had a guy friend for 6 years and everyone of his girlfriends hated me for no reason other than the fact they thought i was trying to steal him and it was all in their head.