First big argument!! Help?

Me and my SO never fight and here recently we've been going at it. All because one of his "friends" over stepped her boundaries. I thought she was my friend too. I just don't know what to do. I apologized to her for being a b**** to her but all she had to say was they were friends and that was never gonna change. BUT THAT WASNT THE PROBLEM. I don't care if they are friends but she needs to that there are boundaries to our relationship and I just can't seem to make her understand that. Has anyone else had a problem similar to this? And what did you do?