I never understand threads like these.
So many people seemingly unable to move from the OW-centred perspective. 'You just want to hut HER'. 'Why ruin his life' 'Revenge is bad'... etc.
And yet, and yet... so many people saying, at the same time - if it was me, if my partner was playing me for a fool and I had complete strangers debating my lie of a relationship behind my back - of course I hope that someone would tell me the truth about what was actually going on in my own life.
The OW is meaningless in this.
So what if OP's main driver is revenge, hurting OW, etc.
The most important - in fact the only thing here really - is that OP has information which could, probably will, make all the difference to how another person chooses to live their life.
This poor guy's life is a LIE.
If I had information that could prevent someone, oh I don't know, buying that used car which a mate has told me is a death trap... from sinking their savings into a business that I have insider knowledge on and know it's going to fold in 6 months... from buying an expensive laptop from a retailer I know is about to go into liquidation... would I tell them? Or would I just think 'Butt out - none of MY business.'
Of course I'd tell them.
She's been cheating on him for a long time. No hasty 'mistake' here. He's presumably blissfully ignorant, and could well be planning to link finances, have children with a woman who is taking him for a nasty ride. What you say, then, could make the difference between a lovely, innocent guy ending up having a family with the right or the wrong person. Ending up as a weekend dad to his children through no fault of his own. You have knowledge that could stop him fucking up his life.
Please tell him. It is the decent thing to do. The OW? Who gives a fuck. She isn't important, and neither is any 'perceived' revenge. Feel it if you like, enjoy it if you like - I'm sure you're past that anyway. Just do the decent thing and tell this poor guy the truth, for his sake.