I have a friend like this. She always used to insist on taking her sociopathic P with her every time she left the house so that he could either let us bask in the glory of his charisma and charm, or alternatively, create a tense and uncomfortable atmosphere, depending on his mood and whether there was anyone present that he respected enough to make an effort with.
The fact that she would call me once a month to tell me about her plans to leave him, and about all the gaslighting and other mean shit he does, made it possible for me to say that, actually, I don't want to see him, only her. After initially being completely open about the fact that I personally think that she should leave him, these days I don't really say a lot of negative things about him, unless I'm agreeing with her about something, and I try to make it clear that I don't judge her for staying with him. She doesn't need me to make her feel bad about that. I keep in regular contact, and she knows I'm here for her and on her side. I also try to remind her of who she used to be before she met him (strong, independent, totally fearless).
It's a very difficult balancing act because if you try to push her to leave him, you can end up pushing her away and then she may have no one to confide in. At the same time, you don't want to look like you think it's OK the way he treats her.
An example of how you can approach it: I once suggested they take a break. I knew he would never agree because he knows her life would be better without him. I didn't actually say this though - she did. She realised it when she thought about it. The fact she came to that insight instead of having me say it probably made it more real to her.
But, yeah. She still hasn't left him, so what the hell do I know.