This is really good advice, thank you. It's gotten so bad between them SD says she fears saying anything to her mom because she blows up over it every time. SD has expressed to her dad that her mom does this and will call her names and pretty much the definition of unhealthy and unfair fights. She has an older and younger sibling (not fathered by my SO), all 3 of them leaving the kid stage. And I'm sure as a mom it's hard to deal with this kind of dynamic change.

Her dad has said if things ever get too bad, to call and he'd come get her (leaving me safely out of it). I think you're right I shouldn't encourage my SD to have a relationship when it's not my place. It will be between her and her mom to navigate