I'd say move on, but the wife at least deserves to be told the truth. Do it in writing if you're too afraid to see her in person, but she needs to know, if he's fooling around unprotected he'll eventually pass her some MST, and if he's fooling around he'll eventually leave her at an age where she won't have much chances to find love again. Saying "she probably has her suspicions" don't cut it, in mariage one can give his trust and expect the best from his/her spouse, and that doesn't mean they should be punished for it.
She's probably having a shitty mariage and not understanding why, trying her hardest to be the perfect spouse and making her own life miserable in the process. Truth makes you free, it doesn't only apply to religions.
Edit: don't tell him you're going to talk to his wife, or have done so. Make it clear to his wife, that way she can chose to act like nothing happened if she wants. And please advise the wife to see a lawyer before acting on any decision she wants to make, and see the lawyer before she even talks about it to anybody else, elders included.
That way you won't be acting out of vengeance, but out of basic human decency.