Roommate troubles

So my roommate was my best friend last year and this year we've kind of gone our separate ways due to hardly seeing each other even though we are still roommates. The problem here is she cheated on her boyfriend last year multiple times because he wasn't here and now that he is she never leaves his side. and I feel like she is an emotionally abusive relationship. He says and does thing a that don't seem healthy for a relationship... And I don't feel she is completley faithful either especially after her cheating last year, but when I ask her if they are still okay and question why she cheated she just says it was random and she loves her boyfriend and always will. Yet still might not be faithful.
I just don't know what to do... Any advice?