I'm sorry but I think it's such an unhealthy narrative to be pushing that she should immediately move the kids away from their dad. If he's cheated on her, he's cheated on her that's one thing but those kids are looked after, they're happy and clearly she thinks he's a responsible enough dad for the fact that even though they've split up he's looking after them.
There's been no domestic abuse, no abuse of any sort and absolutely no reason that those kids shouldn't have both parents. Parents don't always stay together but parental alienation is horrific and certainly not an agenda that should be unjustifiably promoted!