I know how this feels.
My ex is currently dating someone who I deem a mistake to humanity (horrible thing to say on a website like this, huh?) and I found out from her brother that she called him and mentioned a bunch of bruises on her. Her brother and I both believe he must've abused her or something. I told myself that if I found out it was her boyfriend who did it, I would kill him. Unlike you, I believe I will and I would. I do not tolerate any physical harm on girls in any sort of way. Plus, she is my friend, and I do not tolerate people hurting my friends in any way. He is a douchebag who I would more than willingly shove into the ground for her. She lives with him and I keep fearing that he has manipulated her and made her think she can't live life without him.
Confront her. Face to face, alone. Express what you are feeling. Pride has nothing to do with any of this at this point. Complete humility. Even if you sound like a silver screen actor, tell her exactly how you feel to her. It's harder than it sounds, but once you start, it flows out with ease. The reason I say face to face is because anyone can ignore a text, email or written note.
The only other alternative is to completely drop her from your life. Though this is probably the hardest option, based on experience.
Loved a girl, she used me as a test, dated a moron who never has time for her, and no matter how hard I try, I can't lose her. I want to be with her again, but with me leaving for the Army so soon, even if we hooked up, she probably wouldn't stay faithful.
But that's going off topic.
From my point of view and with the info you have given, I believe you have 4 options.
Kick his ass, confront her, keep going as you are (not the best since you are resorting to asking for help about it) or drop her. I'm sure other people would be able to give you more options, but being a practical man, those are the only 4 options that I see that will do anything in this case.
I hope this helps.