B_A
I was here
Kids are smart and perceptive. Waiting to say something until she brings it up might leave her feeling like people, including her parents arent straightforward with her. You could watch a movie together where parents split up and you could ask her how that makes her feel and then go on to explain that sometimes that happens and that it's not anything the child has done. And that adults sometimes dont realize until later they arent happy together.not really I just told her me and her mom were friends she never saw us together as far as I know. It all seems too confusing for me to be talking about.
You don't have to make it specific to her mom and husband but it might be an easy way to drop the seed that something is coming without saying, btw, your mom and her husband are breaking up and you two are gonna have to move.
Be her rock, her steady, straightforward dad.