I find this really common. I've stupidly been in 2 DV relationships and both times the mothers were willing to turn a blind eye and defend and protect their sons.My MIL has never asked me about it, offered consolation or a sympathetic ear. In fact she blames me. Her son is untouchable. I'll never forgive her for her lack of empathy.
If my boys grew up to be like that i'd be horrified initially and then just incredibly disappointed. I would do everything in my power to try and convince him his behaviour is wrong and to seek professional help. I would offer the partner and kids a place to stay and any help i could.
I guess in a situation where the woman won't leave, reporting to it DOCs and the police might be the only way to try and ensure the safety of any children involved.