Speak to Womens Aid, have a good long look at the pages on Refuge too.
Refuge have loads of brilliant info and advice for getting out of these situations safely and for supporting a friend to do the same. They cover all aspects of making a plan in secret - including safe/danger code words, getting important docs out, getting help with children etc.
www.ncdv.org.uk/domestic-violence-protection-orders/
The NCDV is a charity that will sort out a free restraining order for her when she's ready. Make sure it had powers of arrest attached.
Her partner won't know anything about it until its served and if you liaise with the local police they can agree to be nearby out of sight at the time to make sure he doesn't kick off. If you've spoken to the police from this latest incident, keep a note of a contact name and number - hopefully someone from the 'dv' department.
It takes time for someone to be able to leave and its far from straightforward. How you can help her is by doing as much research as you can so you can reassure her and allay her fears as they come up. Give her the facts and info of all the help and tools available to get him out of their lives when she's ready to hear them.
Reassure her that you know its scary and you'll support her whatever she chooses to do. He takes control from her so your job is to ensure she feels in control of the process of detaching.
She might not leave successfully at first, that's quite normal. It takes many women more than one attempt.
Try not to appear a threat to him - be pleasant when you see him. Let her know you're always there.
I helped a friend get out and was their liason point with the police, dv charities and solicitors which helped her a lot. I got her a secret phone, we agreed a pass code that she code remember under severe stress and when scared and we set it up to look like mine so if he did get into it we could pass it off as my work phone.
It contained my number and the detective's number amongst other key support figures. We had a phrase/word so that when we spoke she could let me know if he was there and couldn't speak freely. We also had a code word she could use to tell me she was in danger and need help now. Her phone also contained the what3words app so I and emergency services could locate her accurately.
The biggest barrier for my friend at every step of the process was fear. We reassured her she could change her mind and every stage, that we'd figure out another way of doing things. It was about constant reassurance and giving her the control - making sure she knew the choices were hers and not mine (I'd have had him arrested the first night and the key thrown away).
I provided evidence that went as witnessed by me in the non molestation order.
I look after irreplaceable personal items and important documents for her that she was able to sneak out the house.
It took about 18m in total from when I first found out about him, but only a few weeks from when she reached the point she was prepared to leave which was when we put all the above in place. At all times I let her know I wasn't judging her and made sure she was OK to continue.
If your friends biggest barrier is her fear of losing her children, then one of the best things you can do is find out facts about what might happen so you can reassure her and prepare for what involvement the authorities will want.