Relationship with a best friend and boyfriend?

I've had this same problem. If this guy seriously means that much to you then its ok if you want to take his side. My fiance and I were together all through out high school so we delt with alot of drama like this. If you dont want to be friends with her anymore just kinda give her the hint or straight up tell her "Hey you were a ***** to "Benny" and I dont agree with that." If you still want to be friends with her tell her that you didnt agree and try to work things out. If shes still upset then shes not a good friend and most likely being really immature about it! Hope this helps!