A tough situation for sure. You say that the marriage was 10 years in length, they have a young son, and that she is very stubborn. Hmmm. . . what about HIM? What was he thinking? The problem now is that he is caught! What caused him to put his marriage and child on the line in the 1st place? My biggest question would be what makes you think he won't breach her trust again? And if you were to ask her, I'd be willing to bet that is her biggest question also. If I were in her shoes, I can't think of one thing that someone could say to me to undo what was apparently a period of time of deception, by the one person I am supposed to be able to trust. Her husband might be able to save the marriage, with therapy (more for her sake than his) but I would think he would need to make any online activity known to her and even then the trust is broken. I believe in that situation the only thing I would believe would be a program that recorded every keystroke made on the pc.
For some reason, I have ran across a lot of people who feel what they do on the computer is not the same as if they were out doing it in person. The easiest way to know if you are doing the right thing is to ask yourself this: How would I feel if the other person were doing this? And then act accordingly. We live in a society where often people act and then they think and no one wants to face the consequences of their actions.
One thing I learned along time ago is this: for some things, there simply are no answers. If this is the conclusion the wife has drawn it may be time to move on...