Quick question

How do you feel about someone detaching themselves from a friend when the friends relationship issues start to become a burden on you? My friend has been with the same guy for years, and he is toxic! Very bad for her. All she does is cry and complain about what he does to her. And as a friend of hers for over 5 years, I have supported her and been her backbone with this dude. She comes to me all the time about how he does this and that and how he just won't change, but she goes back. But I don't want to be that friend that she can't talk to either. So I listen to her vent about the same thing over and over again with no changes after the conversation. She complains to go right back. Now this whole situation is draining me because she's sad 24/7. We can't go out without her thinking about who he with/ what he doing, etc. what should I do?? Help 😩