You could do it, but I just don't think it would do any good. The OP that gets involved with a married person with kids don't put much respect on morals or values or marriage. If he had a W, then maybe that would do something, the fear of getting caught, but you said he isn't.
I did that once, emailing the OW, and it just didn't make me feel any better. Not really any worse, just didn't do much either way.
I think it's up to your W to not contact OM.
And I almost forgot to say, you have been handling this so well!!!! I think it couldn't have worked out better. I'm so glad you didn't just move out and almost give up or whatever.