I think you’re doing all you can, but she’s ultimately a grown woman making her own choice and it’s not on you to deal with the fallout.

I had a friend like this who would get back with abusive exes time and time again. I supported her constantly, but it was exhausting and really took a toll on me, and I wasn’t even in the relationship?? I should have set boundaries, and I think you should in this case.

Be clear about what he is: abusive, and be clear that he will continue to treat her in an abusive way. Support her as you would any friend, but not to the point it affects your life. Set boundaries about what you are, and are not, willing to offer her in terms of that support. Again, it’s her choice here to re-enter this situation, knowing full well the consequences.