Sharing Our Innermost Thoughts

Okay so my friend has been dating this guy for over 3 years, they’re currently in an open relationship but this only happened recently. Prior to that, he had cheated 6 times and she had forgiven him every time, when he reached the 6th time he asked her for an open relationship (basically an excuse to get away with not being faithful, I have nothing against open relationships but if he wanted one he should’ve said it from the beginning instead of promising to stay faithful and swear he’d never cheat) She doesn’t want an open relationship but he told her that she either accepts or they’re breaking up ( basically he doesn’t really want to put in the effort to work through the problems, he’s never shown to care much about whether they’re together or not) She accepted cuz she doesn’t want to lose him because she loves him, she says. She even told me that she didn’t mind if he loved her or not as long as she’s in love she’ll love for the both of them and hold onto him. When she said that I had to say what i thought which is that she was being very stupid and that i couldn’t comprehend why she was putting up with all of this for so long, she got mad at me and said that I’m jealous and that I don’t want to see her happy. But the thing is I can’t understand how she can say she’s happy like this??? she literally comes crying to me every time they fight. They’re not happy with each other, their relationship is the most toxic I’ve ever seen. She’ll literally post ig stories crying over him and idk how to tell her to pls stop for her own dignity, she’s just embarrassing herself. I don’t mean to be mean about it, it just upsets me bc she doesn’t deserve this and she shouldn’t cry and beg for such a piece of shit to stay. She’s already lost friends because of this, she won’t lose me bc I’m not gonna leave her now like everyone else however it is stressing me out a lot and I’m starting to get tired of putting up with her snapping at me every time I dare to say my opinion. What should I do?