No, don’t say anything to her. It would give her good motivation to say you or your dad threatened her.
She’s a big girl, she’s gotta hitch her pants up and be an adult. Stop trying to “save” someone who is ripping your family apart.
Deal with it by being a man. Tell your Dad you want to talk to him, validate that it must be hard to sit back and say nothing, but that is what you expect him to do both for you, and your son. A small dose of “if you say anything out of line, it might affect how much custody/time I get to spend with my son in the future” … that might be enough for him to respect your wishes.
Not sure about the US, but the pathetic family court in Australia very rarely gives equal custody if there’s been any allegations of aggression, verbal abuse etc. - even if they are false allegations. Women know this and pull the DV or abuse card even if what he says to her is true. If your Dad gets up in her grill and gives her a big serve, she may falsely claim domestic abuse and suddenly you can’t see your son.
