I was in a relationship like this once (when I was about 18). Luckily I got out before I found out how abusive it would get.
I think what's more painful than knowing your daughter is getting abused, is knowing it doesn't bother her much. Whenever my ex-so continued to horse-play at my expense, I lit a fuse, and on one occasion I kicked him out of my house.
I believe that as a mother you should show concern for her feelings and well-being. This means asking her how she feels about these occurrences and teaching her that she should set limits. You should also tell her the signs of an abusive relationship (hopefully she's start noticing each one, and it might bother her some more). If he ever does cross the line, she'll think "hey, my mom was right, I need to get out
right now".
And lastly, don't alienate her. Other people already mentioned this- if you set boundaries and limitations then you become "the bad guy" and your daughter won't want to listen to your advice. The best thing you can do is provide information, support and guidance.
And those are my 2 cents
