I don’t know what to do about my friend

My friend and her boyfriend have been together off and on for years. Over the course of this time, she’s had multiple different people reach out to her telling her that he was cheating on her myself included. Everyone had proof and she would confront him and he’d lie about it. She always talks about how he says he loves her and how he talked about fighting for her while he’s still going behind her back. I’ve tried over and over again to snap some sense into her but it’s like she isn’t hearing me. What should I do?