You have to listen, tell her to talk to you whenever she needs to.
You have to tell her she needs to seriously think about getting out.
Tell her to get a bag together, put documents bank statements, passports, birth certs, insurance, car ownership, anything and everything official. changes of clothes fo her and DC, and keep that bag somewhere safe. This is something you could do for her, hold her flight bag.
Tell her to squirrel money together and if it kicks off, she can just pick up her DC and walk out knowing she has cash and clothes waiting for her in a safe place.
She ought to speak to Women's Aid, they can advise her on refuges, hostels, benefits etc and can help her with the legalities of keeping this man away from her in the future.
You could keep notes on the incidents of abuse, it has been known to help in DV trials.