You say he got himself into a situation.... as if he sort of fell into it by accident. Yet he went online and deliberately sought out no strings sex.
My parents split when my brother and I were just a bit younger than Urbane's kids and I would agree, it was easier in that I have no real memories of them together - it was my 'new normal' almost immediately.
If he's prepared to make some sacrifices at work there's no reason why he can't go for 50:50 care of his kids. I know I would be devastated to only have my ds with me 50% of the time but it doesn't have to be a question of every other weekend.
He also has the option of asking his wife for an open relationship. Who knows, this kind of balance of domestic life and fun might suit her - either for her to indulge in as well, or for him to do with her knowledge. There are MNers who live in these rather uneven open relationships, i.e. where only one party is playing away - but if everyone's happy it can be made to work.
However, he's denying his wife all of these choices by making one for her, the one where he gets to have his cake and eat it. The one that humiliates her the most.
Beyond telling him that, and refusing to do anything that condones the relationship (discussing it, providing alibis) I'm not sure what else you can do. If you think he might be practising unsafe sex I think you have a responsibility to tell the wife for her own health.