Have a friend in same circumstances who did this. Despite my advice not to, contacted the OW on facebook, who then phoned her. My friend then told her everything, including the fact she had a 2 year old 'love' child. All apparently with good intentions (she needs to know, doing her a favour, he can't get away with it....you know, none of the things which were necessary before, when they'd been together.
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Anyway it backfired spectacularly.
When she told her what had been going on, instead of being pissed, the wife was sweet as pie (at least outwardly anyway). She was aware her husband had form for cheating, he'd done it once before, and although she was suprised to learn about his secret child, she was such a nice person, that she expressed concern about my friend's child and the possibility of him not knowing his father and siblings as he grew up! She said she thought my friend had been treated terribly, offered support, shared how disapointed she felt about her husbands behaviour, and offered to meet my friend with the children so they could get to know each other. Then she asked her husband to leave whilst telling him he ought to reconcile and make things right for the sake of all the children!
I can not tell you how much this pissed my friend off. Over the years of the affair, she had built a picture of what a cold, sexless, unaffectionate, cow this woman was, as described by the lying cheating man. In her mind she had become an adversary, the thing keeping them apart.
What she really wanted when she so altruistically contacted the wife, was for her live up to the picture she had in her head. She wanted her to rave and bitch at her and have a spectacular showdown, in order to make it easy to hate her and justify continuing 'fighting' for the guy, but what she saw was that the wife was very like her, just a lovely woman who had been in love with and deceived by a scumbag. The more I kept saying the wife sounded lovely the more pissed she got. It was suprising how similar they were temperamently amd personality wise, both strung along by this guy.
At this point I was prepared to lose her as a friend as I thought her behaviour was awful. But several days later, realising th truth of the situation, she confessed she knew she had done something really horrible and felt ashamed.
Long and short he's gone back to the foolish wife, but its worse for my friend now because she knows she is in fact quite lovely!