I'm concerned about my best friend's relationship with her boyfriend.
Why are you concerned with your best friend's relationship?
Her boyfriend is violent and I'm afraid that he's abusing her.
I'm happy that you're concerned about your friend's safety. You should talk with her and see if she needs help. She may not know how to get out of a relationship that is unhealthy and dangerous because she's afraid.
Do you know what I should do?
You should talk to her and tell her how you feel and that you see what is going on. Also offer a safe space if you can for her to talk about what might be going on.