Ultimately, you have to realize that he is not your man. No matter how you feel, he has a girlfriend that I'm guessing you both acknowledge AND know personally. You are in your feelings because you've decided that you want him or at least don't want anybody else to have him.
This new girl isn't an issue because you can monitor the relationship and you know she's too new to make any real difference in his life. His girlfriend, however, is a permanent fixture and you don't like that.
Maybe you need to back off of that friendship a little bit to get your feelings in check. The fact that you had a big fight about his time with his girlfriend implies that you two need to re-evaluate the relationship you have. Because from the cheap seats, it seems like you two have an inappropriate one.