I would agree that we could debate the point of whether the father should have exposed the mother's deception to his the kid. I believe there are valid points to both sides of that argument, and saying such will probably have a lot of people on both sides who think I'm dead wrong.
What I do not think is disputable is the fact that what Mother did was far worse then the Father exposing it. By extension how can it not be? If the act wasn't that big a deal, then no one would care much when/if it was exposed. The only reason we are discussing the act being exposed because of the horrendous nature of the act to begin with. On no world, is the Dad more at blame for letting the kid know what Mom was doing over Mom DOING was she was doing.