I've been friends with a girl, who I'll call Samantha, since high school. Four years ago, Samantha started a relationship with a lesbian, who I'll call Alex.
(Alex is a genderfluid person and prefers they pronouns, but identifies as a lesbian. This does not relate to the question itself, just a guide about how to talk about them.)
I've known for several years that Alex has been emotionally and financially abusive to Samantha. I have rescued her from situations before, and Alex knows, but I appeared to move on. I continued to talk to them and be around them.
After months of debate that I've supported her with, Samantha ended the relationship and gave Alex time to gather their things and leave. Alex even posted on Facebook that they'd hit Samantha and were being broken up with as a result. It was the most permanent break up I could imagine. I cut off all contact with Alex on social media afterwards and have not spoken to them since.
As the weeks passed, Samantha talked to me less and less, even though I checked up on her frequently. Alex continued to live there. I feared for the worst.
Then the worst happened: they're back together.
I love Samantha so much and want her out of this relationship. I know I can't do that, but I want to be as good a friend to her as I can. How do I hang out with her now? How do I still be her friend and around her? Alex knows that I want nothing to do with them, and I have not unblocked them. I hate them and have hated them, but I've been nice to them for Samantha's sake for all these years. Now, though, things are much different. Samantha believes/knows I cannot be around Alex any more.
tl;dr My friend got back with her abusive partner, who I've cut contact with. How do I hang out with her?
edit: extra context, Alex and Samantha are very poor, I live with my parents and can't host them often, we all identify as white, and we live in the southeaster United States.