"We need to respect that Mommy feels this way, and that this is what she wants. I don't want it, but it's OK for grown-ups to disagree with each other sometimes, even about important things. I still love her very much, and I know you do too, and she will always be your mother and we have to respect her. Do you understand?" (or some such).
I think that is too much for now. Assuming his account is accurate, I think it was fine and I am ultra sensitive on this topic.
