Okay, so my friend has been with her boyfriend for a little over 4 years now, and they aren't exactly the most functional couple. He is very controlling and won't even allow her to have friends because he either gets extremely jealous or is afraid that she will leave him. I'm not really sure. But he is clinically depressed and often gets very angry or emotional. Sometimes threatening suicide or even attempting it. Especially when he feels like his relationship is falling apart. He is insecure and has low self esteem, and if he loses while playing video games, he gets so overly pissed that he scares her. She also has a few contributing factors to this however. She is openly bisexual, so no matter who she hangs out with he sees as a threat to their relationship. She also has claimed she is poly-amorous, but I have since convinced her that it isn't a good idea. Pointing out the problems that she hadn't thought through before. So now she wants to stay in a mono relationship with her boyfriend. They both have a ton of issues that they need to work out on their own, and I believe staying together will only prevent them from working through it. I also think that his over-dramatic reactions and threats are his way of controlling her, causing her to run to his side every time. I don't know the full story, but from what she's told me they are both in a very bad spot, and neither are willing to try and get out. I don't know what to do to help her anymore. And I say her because the guy pretty much hates me at this point. Do I just ignore her? Or do I try and help guide her? I'm lost at this point. And please, no one attack me for convincing her to be mono rather than poly. There were a lot of things she didn't think through when she started identifying as poly. Like how it would hurt everyone else in the relationship. Humans might not have started out mono, but it has since been instilled in our society, personality, and over the years our brains have been wired to seek a single partner rather than multiple. So please, just logical answers on how I can help her. They are destroying each other from the inside out.
You can't really do much if she wants to stay with him. If she was directly asking you for advice on how to exit this relationship, then maybe you could give her some input, but it doesn't sound like she wants to leave him.
Best option is to support her as a friend and keep your opinion to yourself. You might push her away if she thinks "you want to break them up".