You do have a point about the kids seeing what's been done, but maybe them seeing it is what should happen. If their mother is a cheater, she is most definitely not setting the right example for the kids and so their dad has to do it and if anything, he may have done the very thing that his kids need to learn. When something over the line far enough wrong is being done, you do not just sit there and let it continue. Instead, you blow that whistle on the guilty person especially if it is something illegally wrong that only hurts other people. Remaining silent will only make you an enabler.
God bless you and the man and his kids always!!!
Holly