What would you do?

The kids are young, and this is family, so cutting them off is not something I'll do.

M cheated on his wife F. They separated for a couple of weeks, and against everyone's better judgment, she begged him to come back. When he did she expected everyone to support her choice because "only we know what really goes on behind closed doors", "this is our relationship", "this is the healthiest thing for the girls". Mind you, it is the kids' argumentation that the reason everyone swallowed their feelings and had to give M a chance and treat it like "the mistake" that it was.

Come to almost a month later, M says to F he just doesn't love her anymore, he doesn't want to be in the relationship and even though she cried for him to make it work, he leaves but says they will co-parent.

Now F says no. He is not EVER allowed to see the girls anymore and to get a good lawyer because she's filing for sole custody, and in the meantime, she's refusing him to see the kids. Not only that but when we tell her to not use the kids as weapons to hurt him like he hurt her and a parent/child relationship should not be determined by THEIR relationship she calls you a traitor and now the same person who forced everyone to accept and be ok with him is giving an ultimatum that you are NOT to be ok with him at all or she'll cut you off. What kind of mind fuck is this?

I'm going bananas, the kids are crying for their dad and regardless of what happened with them, they are both excellent parents and he deserves to see his kids as much as he does. What would you do?