I'm so worried

My boyfriends best friend just hit his girlfriend last night while he was drunk, they decided to stay together. My boyfriend and I don't know what to do, we both complexity lost respect for this guy and I want him out of our lives, but it's his best friend I can make him kick him out. And I'm so worried for her. I feel like this will be a constant struggle, in our relationship now. I've been so anxious about everything. Anyone else been in a situation like this, what should we do?