She's in an abusive relationship
I planned to ask this girl out that I worked with, but the day I decided to do so, I found out she was engaged. I immediately abandoned the thought and put everything off as mistaken friendliness. Then later, out of the blue she tells me how much I mean to her. Confused, I ask her to explain. She says that her fiancé has treated her bad for a long time and she's finally started rethinking everything and was considering leaving him for me.
We spend the next couple weeks talking a lot and hanging out, and in this time I find out they've only been together for a little over a year and he proposed right away (I'm guessing because his ex left him after 5 years and wanted to try to prevent that again). Eventually she says she decided to stay with him because he supposedly was getting better CONVENIENTLY as soon as she started rethinking things and she said she thought she owed him.
She wanted to stay friends, so I've still been hanging out, but then when hanging out with just her friends, they started revealing things she never told me like how he always yells at her and threatens any guy that gets close to her and he uses steroids and even told her he was going to give her some for her birthday (which he's yet to give her anything and actually made her cry on her birthday) and how he constantly cheats on her.
They've been telling her the entire time how horrible he is and how he cheats, but she always blows them off by saying she can't prove he cheats or trying to justify things, etc. Lots of things that ask sound like denial. I have no idea what to do. Even putting aside my feelings for her, I can't stand a good friend going through this and hurting themselves. What do I do?