Don't - you will get over it faster, with less drama and more dignity if you rise above it.
I know it is hard, but their relationship is their deal and your DH is responsible for you. She is zero.
Another reason not to do anything is that if you do say something and it goes horribly for him or for the DC, there's always the feeling that you might have had a hand in the outcome. Right now you are completely unconnected to their lives. What if you tell him, there's a massive row and one of their DC fails an exam the following morning because of it? Or the DH's mother is dying the same day?
Even if you feel massively righteous about how he will 'thank you', you have no idea about their lives, the timing, the DC, anything. You could be involved with causing innocent people a fuck load of trouble. And, yes, I know the fault lies with the cheaters, but you are responsible for your actions.
Tell us, scream at DH, tell your friends, but don't be involved with the destruction of a family - however, tangentially.