Worried about my friend

So my friend has been with her boyfriend now fiancé for 12 years he’s been in another public relationship for 3+ years with another girl who he obviously won’t leave alone. They both know about each other. He has kids with my friends and also has had 3 kids with the other woman. For the longest the woman didn’t know about my friend but feel deeply in love with this man. He’s a great father and he’s good to both of them despite not being able to let either of them go. I think he uses my friend as an atm as the other woman everyone says he treats and cherish her like a goddess. I tell my friend she insane for still wanting to marry him because he will not let that woman go and she is making herself look like a fool. And as of right now she is not talking to me. She gets a kick going through his phone once a week and either him and the other girl are arguing or she finds something she doesn’t like.