OK. I think I have my head around this now I have decided to ignore all but your last few posts - the title and the OP and the following ABCDE stuff was all bonkers!
But what it boils down to is this: your ex had an affair with OW, she maintains he didn't, you kicked him into touch because of it, she lied to her DH and they are still together. But he is your friend and you want him to know because if the boot were on the other foot you'd want to be told.
He may not want to know. He may know, deep down - the fact that you booted your ex out over this apparent "non-affair" might have been a big enough clue, don't you think? He may have either chosen to bury his head in teh sand and carry on with the fantasy, because it's easier than dealing with the reality of the situation; or he may have decided to give her another go.
However - if this "non-affair" is still going on and he doesn't know about it, he probably should - it's better to play the game with a full hand of cards - but if YOU tell him about it, you may lose him as a friend. Is that a risk you're prepared to take? Messengers frequently get shot (I've been there) and you have to know that it is a likely outcome.
Is there anyone else who could let him know or do you feel it has to be you? Because I suspect that if you feel it has to be you, then you are a wee bit guilty of a spot of score-settling, disguised as do-gooding. Understandable - but you need to take a step back and work out exactly who you are wanting to do this for.