"If she sustains the lie, she is doubling the wrong she has done to the guy's wife."
Why does not spilling the beans "sustain a lie"? The lies have already been said.
Sorry, but it really is not up to another person to stick an oar in about whether or not he is chosing to lie to his wife. That simply is not anyone else's business.
If someone CHOSES to get involved with someone who is already in a relationship, then they ACCEPT that there is going to be an element of lying going in from the very beginning.
When things start to go wrong, it is obiously very convenient to suddenly become a moral crusader, lecturing the errant husband on what he should be doing. Sorry but that is just so hypocritical it is quite laughable.
What the wife choses to do is up to her - she may be going on incomplete information - that is not the responsiblity of anyone else.
The other woman did not get into this as some kind of marriage guidance/moral crusader/advocate of honesty in marriage.
"Assuming the OP is single, he is the one who has done the most wrong, by a country mile."
I disagree - IF I was ever in a situation where I became involved with a married man, I would consider myself to be EQUALLY responsible (even allowing for the bullshit that would no doubt be fed to me about "my wife doesn't understand me etc, etc)
"The person they lied to seeks corroboration from you, and you say nothing." That is not the case - the corroboration is there. She knows what happened. Does the lover really need to feed all the rest of the information to the wife, is that really necessary?
Suppose it was the man telling the wife everything about the lover without her knowledge or consent - that would look really weasly - like he was trying to get himself off the hook.
"It is also not gossiping if she is responding to a very hurt and damaged woman's need for information."
But she is just adding insult to injury here! First she has an affair with the husband and next she condescends to be the shoulder to cry on for the wife!
To be honest, if I was the husband I think I would chose to be on my own!!