The estrangement between the spouse that left and the kids just seems to add more pressure in their world. I agree no to speak badly to kids about H, but it is on him to fix his R with them. It's hard to watch.
My D's don't really talk to my H and I know it bothers him alot. Yet, when I update hm and ask for his input with regards to them he says I'm pressuring him for an answer right now and that I want him to fix it. I have just backed off and now ask if he'd like to hear what's going on.
Too bad he'll be missing out, but he's made his bed (so to speak).