My friend is in an abusive relationship and I don't know how to help her?

Tell her mother everything that has happened, she may dislike you at first, but in the long run it will help her. If you are worried that Alex is a danger to the public, talk to her mum, your friend and his dad if you should call the police or a therapist. Most of all you should try and be supportive, i understand it is difficult as you have already tried to over a year. If it gets any worse i would go straight to the police, or one day it could end badly. You need to show your friend what can happen if something does happen, she doesn't want to end up in hospital, so encourage her to see what will happen if she stays.