I asked this question here quite recently.
I decided in the end not to tell the OW's DH because I was aware that my motivation for wanting to do so was not because of some sense of moral obligation to my fellow dupe, but because I wanted to harm the OW. A poster here reminded me that he has already been treated as collateral damage by my DH and his wife, I don't need to join their club and make him feel even more worthless and dehumanised as a way of trying to ameliorate my own pain.
I think that if our places were reversed and he knew and I didn't, the last think I would want is him telling me as a way of punishing his wife. He'd just be one more person using me as some kind of puppet in their game of My-Feelings-Take-Priority-So-I-Don't-A-Fuck-Who-I-Hurt.
I think that if - if - you were able to be sure that your motives for telling were purely driven by concern for the other betrayed spouse, then you should think really carefully about how to tell them in the kindest way possible. This may well be telling the OW that she has 24 hours to tell her DH herself before you do it. It would also involve being prepared to answer his questions, and handle his distress, so a face-to-face meeting somewhere safe and neutral, definitely not an anonymous email etc. This is all very hard to do, both practically and emotionally. You're shattered into tiny pieces yourself, barely keeping it together - it's a big ask to then manage someone else's pain too.
And of course, he may not thank you for the information. He may become angry, accuse you of lying, or blame you - none of which you need when you're reeling from the discovery that your DH has been unfaithful to you.
I suppose what I'm saying is that you have to protect yourself first and foremost, and telling the OW's DH categorically will not help with this. I guess it might give you an immediate sense of satisfaction or vengeance, but then we're back to the fact that you're treating him with similar disregard for his feelings as his wife and your own DH, and that's not a good thing.