DO I INTERFERE OR JUST LEAVE IT

OK so i have a friend who is in a relationship with a boy who is pretty controlling. I was talking to her mother and she said that she heard from a romour that her boyfriend hit her... and if she where to find out if that is true she was going to go off her face at him So i wanted to catch up with her the other day and seen there was a bruise on her jaw, I asked her straight out "does your man hit you?' she denied it at first then i told her that i knew she was lying and she finally confessed, "yes he has hit me once" she told her mum that she walked into a door.... So my question is should I tell the mum or just keep my noes out help please.... i am worried about her...