Assuming the child isn't in danger then it's a tricky one because break her confidence and she may break off your friendship- thus losing the only lifeline she has and make her reluctant to open up to others. OR - she might be secretly thankful you speak up.
This is what I would do: speak to Womens Aid. Do some research, find out everything you can about what will happen, could happen and what choices and options she will have at every step of the process of leaving.
Leaving an abusive relationship is terrifying, more so because its a step into the unknown and uncontrollable, so your role is to do the fact finding for her so you can inform and reassure.
Emphasise her choices and options along the way, she will need to feel that she is in control.
Then be there for her and encourage her to take steps when she is ready and reassure her that it's ok to not be ready, to have doubts, to be frightened, to feel guilty.
Both women's aid and refuge have brilliant advice online.
You can help her put together an escape plan, give her a secret phone...you could be a point of contact for services which are there to help her. There's lots you can do and lots of info you can find.
Keep being reassuring and gentle encouragement, emphasise her choice to stop the process at anytime. And if she names a worry - find out about the reality on her behalf.