This is a complex issue. Your friend is probably caught in this cycle where she thinks that this normal, and she/he afraid to leave. If she/he has history of abuse this makes it 10x more difficult. Try to be there for him/her. Be very supportive. But, support won't help him or her get seriously injured or even die. I recommend coming up with a safety plan. If he/she significant other goes to work, go to the cops. They might put her/him in protective custody and get the other person locked away.