My mother stuck it out for 8 years. She says now it was like watching a slow motion car crash, and she could not understand why I didn't see what he was up to. And she only saw the obvious stuff.
It was very hard, because he drove a huge wedge between my DM and me. Going and sulking in a different room if I was happily chatting, accusing them of looking down on him when I couldn't see any evidence of that at all, not talking to anyone at dinner time when staying with them, screaming at my DM that 'you must think I've got 'fucking mug' tattooed on my forehead' because we'd gone shopping together and he'd had to look after the children with my SF. It was hell. I'd think we'd all had a nice day and then we'd go up to bed and a torrent of complaints and imagined slights would pour out of him and he'd EA me into the ground.
So I saw less of DM and it was strained when we did.
But! I got out, and went straight to her. It was a slow process, waking up to what he was doing, but one day it abruptly snapped into focus and I got him to leave the house and changed the locks. Then I went to mum's. I told her everything and she finally understood it all.
Now, 3 years later, we have a better relationship than ever. She herself has come on massively in understanding abuse, and has started training as a volunteer with WA to help other women in my situation.
There's lots of reading you could do, or even go to the Freedom Program. The best book is Lundy Bancrofts 'why does he do that?' Isolating a victim from their family is a major abuser tactic, and the awful thing is that quite often the family become angry with the victim, as they are not seeing/understanding the whole picture, and so they become complicit with the abuser and MAKE THE ABUSE WORSE by inadvertently joining in.
The best thing you can do is to tell her you love her no matter what, that you think how he treats her is wrong and awful, and you will help her however you can if she wants to ask for any help. That's it. No recriminations or accusations on her for what's happened, just that one thing. Think of it as a pure lifeline. If she's going to get out, she has to trust completely in you that you love her despite everything. That's all you can do. Be there.