Sounds like the kind of girl how is only attracted to guys that are controling because they feel they need that, either becasuse of the way they were raised or the way there mother has been treated in front of them there whole life. You are just in the " Friend Zone" even if she leaves him for good she wont come to you, sorry i just dont see that happining. We all have one point in our lives where we see a opportinity to be the White Kinght and save the day. When that situation arises you have to assase the situation and decide if you attacking this guy is going to make any diffrance.
Lets say you jump him and fuck him up, there are two senarios that come from this.
1. She thanks you, and either falls for you ( unlikely) or go's about here way to another dickhead.
2. She gets mad at you and comforts him and then does not speak to you anymore.
Either way you get in trobule so I would make sure that you are pretty positve of the outcome before you act. Expect the best prepare for the Worst.
If you do jump him, dont be a pussy no sucker punches no chairs or pipes or shit. Walk up tell him he fucked up treating her like that and then Pound his ass. Dont stop hitting him even if he falls get full mount and just keep hitting until you get pulled off or he goes limp. Also for the love of god dont be like almost every kid i have ever seen in a street fight. Keep your Cool look for openings and dont just throw haymakers work the jab and follow up with hooks and Upercuts( if you can do them corectly, not a Mortal Kombat uppercut)
In any way you choose i wish you the best, let us know how it goes.