My friends 'bf'

So I work with my friends boyfriend. He botched about her all day long saying how she's a horrible girlfriend and whines and cries for advice from people. They fight all the time. Now after everyone telling him he should break up with her, he's telling everyone he has and that he's single. He acts like she's the crazy bitch ex. And has stopped talking about her besides a comment here or there about how she sucks. They are still 100% dating. I've told her what he does but she sorta just....ignores it? Sits in denial? On occasion I get involved in Co workers talking about him and how he's hot for other women at our work and his flirting. Should I maybe mention he has a gf? Or Stay out of it (which is what I've been doing kuz his gf doesn't seem to care what he's doing)