I'm not sure I have advice for your friend, but I have advice for you. Just be there for her, if you care about her.
I was in her situation a couple years ago with my current partner. Her situation sounds very similar to mine with the exception that my children's father is a very good father and cares for them very well.
I started saving and stashing money here and there in case I ever decided to take our son and run, dollars and change for payphones and buses. I established a code word/phrase with a friend so that if I couldn't get away, but could call, they would know I needed help getting away. It never came to that, but I took our son and left him and lived with my mum for four months.
I think it would be a good idea for her to save some money, if she has time and her situation is not so volatile that she is in immediate danger. Maybe you could be the friend she runs to or who could help her run, if she needs to. If you can't help her, there are women's shelters to turn to.
Also, should she decide to leave him in the future, I think it would be wise of her to watch everything she says and does. She shouldn't have loud verbal or physical altercations with him and especially not start them. She shouldn't say hateful things to him. She should be a "model citizen." Don't give him any ammunition in court for why she's a less suitable parent. She should conduct her life as the loving stable mum that she is and try not to get caught up in any drama he may try to stir up. In the meantime, she should be setting up a support network and making contacts so that she knows where to go and who to talk to if she needs to leave and needs shelter, food, clothes, public assistance, job training, legal advice, or whatever else she might need to support her child alone.
My partner never changed and won't go to counseling. I'm sure her husband will never change. So she needs to go to counseling herself. She can't change him, but she can help herself to change and to change her outlook and her life.
This is a difficult situation and I'm still in the middle of something kind of similar. I would just suggest for you that you listen to her and help where you can. Don't badmouth her husband. If she wants to talk and go to counseling, she must, on some level, want things to work out with him. She's probably heartbroken that it has come to this. She probably still loves him and it hurts to hear people badmouth your husband, whether or not it's deserved. Also, should she decide to stay with him, she might feel as though she can tell you less and less, knowing you "dislike" him. She might feel bad about negative comments about him, and she may even take it to reflect on her because she's with him, even if they are true. So, just listen, offer support to her. In the end, what she does is her choice. You should be there to support her choice, not influence it. Later on she could turn around and blame leaving and all consequences on you, as if you pushed her into something. I'd say feel free to talk openly with her, offer her options she may not have thought of, let her know you are there for support and help, but let her make the decision (or not make a decision) on her own.
I'm glad that you are so concerned for your friend and are willing to seek advice on her behalf. You are a good friend to her! She's lucky in that aspect of her life :-)
I wish her the very best of luck. She has a long road ahead of her.
Best wishes,
A.