The longer this post goes on the more I'm in the camp of just breaking it off with your gf.
There comes a certain point in time where you need to cut the bullstuff when you're dating someone. People try to tell you nowadays that having to cut off friends for an s/o is ridiculous, that you don't have to change for anyone, and that if you have to do either, you guys don't mesh well.
That is bullstuff. Sometimes you need to change to make the person you love happy. Compromises are an important part of any relationship, I bet the same people who say the above would agree. Part of being an adult is knowing how to cut toxic people out of your life. This Brandon guy is, or should be, toxic to your gf. He is emotionally manipulative, he has disrespected her and your relationship and the man she made a commitment to. Her continued contact with him even before he put his hands on her was a tacit encouragement of said behavior. She did not put her foot down before any of this, and she is still not putting her foot down by continuing to "be there" for him.
forget that. I've given up friends for an ex. But that's because the friend was doing things unforgivable and ruining my relationship. This is one of the times where it's friend or significant other, and your gf is actively choosing friend over you.
Imagine, if you will, you had a female friend. One that constantly talked about how much she wanted to jump your bones, suck your rooster and all that stuff. One who stuff talked your gf to you constantly. I'm sure your gf would have an issue with it, wouldn't she? I loving promise you she would. And despite her voicing her concerns, you keep talking to this girl. And THEN, you loving go to her house by yourself one day, for what? To play with her loving cats? Get the forget out of here. You know damn well you didn't go to her house just to play with her cats. All those messages about getting your richard sucked finally got to you and you went to her house so you could see what would happen. Oh, and she ONLY put her hands on your crotch.
Yeah loving right. She sucked your richard.
I hope I made my point clear here.