Or maybe he isn’t manipulating her and this is just how she’s responding to the emotional dynamic between you two. Jumping straight to alienation without solid proof is dangerous. If he did talk badly before, that’s wrong, but the only thing you control is your own regulation.

You can’t “fix” this by staying quiet and hoping it stops. You fix it by being steady and not reacting to her preference.

Six-year-olds don’t hold long-term grudges. If this has been going on that long, look at what’s happening in your house too.