I wouldn’t get involved, you don’t know how people can retaliate and since you’re roommates you could just end up putting yourself in a hostile environment if you still have to live with her. I’d say let her tell him if she wants to or try to convince her to break up with him. I wouldn’t get involved beyond that, especially if you’re friends with her and not him.
I know a couple where the girl cheated. I want to tell him so bad! It happened in 2020. I still feel bad he doesn’t know. he has 2 kids with her now. I still feel conflicted. being the bad guy and breaking up a family isn’t good for the kids… she also hits him gave him a black eye last week.
I definitely think you should tell him, but also consider what the living situation would be like for you. If you feel like she’d become bitter and make your life hell for telling, then I’d find some other avenue. Like the top comment said, try getting her to break up with him. You getting involved might make your life terrible
One of my best friends cheated on her boyfriend, who I did not like because he forced her to have an abortion even though she wanted to have the baby. She texted me one day and said if anyone asks she’s staying with me at my “new apartment”. She was with some guy in the city she told her bf I just moved to. But I really don’t give af bc it serves him right. He also has stalking issues and is creepy as hell.