I am 17, so let me tell you what makes a bad boy so attractive : the fact that your parents won't let you see him. I understand you don't want to loose your daughter and you don't want her to get hurt, but there is no easy way out of this. If you want your daughter to stay, you have to let it go. If she stays and you say nothing, she is still in the relationship and potentialy getting hurt, I know. BUT, if she leaves, chances are she will get even more hurt. Not expressing your concern is not easily done I imagine, but if you keep telling her how much you disapprove, you will loose her. So you're probably thinking : "am I just supposed to shut my mouth and look at my daughter get hurt in an abusive relationship?".. Well, for the time being, yes. What you described sounds terrible but there is room for escalation. You just hope that she will have had enough and leave. If it does escalate dangerously, call the police. Another thing you may want to try is talking to her friends. She is more likely to trust her best friend telling her she is worth so much better than a cheater and control freak than her mom who "just doesn't understand". Last method (you may not like) is involving her younger sibling. You said you don't want to affect him with all the drama because he is too young but he most likely already understands a lot of what's going on and he already has an opinion about it. He should, with a little push from mommy, go tell his sister he is worried about her, that she looks sad, anything a cute 8 year old can come up with. It will surprise her that a young child is concerned (which is probably very true on his part anyway).
Conclusion : don't nag, cooperate with friends and get your son to speak out.