You have to be careful here as if he knows you are helping her he could get even more violent with her and with you as well. I had lived with domestic violence for nearly 20 years so I know how these men work.
You could get here some information from womens aid the number is in the phone book. BUT the key thing here is what does she want to do about it?
Also there are child protection issues with regards to the child. I would call the police if I heard him at it.
Women in this situation need support and advise from people they can trust.