I am using Arria's post as the example ONLY because it was the more previous post to mine. That having been said...
Having been there it may not be a matter of heartless... after all, we are only getting one side of the story. What lead up to her "cheating?" Were there problems before that she attempted to bring to the surface that he wasn't willing to discuss? Was he meeting her needs both physically AND emotionally? He is condemning her for behavior that he is in some fashion enacting himself (at least from her point of view)?
Some of these posts are instantly condemning someone we don't know based on very little, one sided, information. And even if new1 tells us the FULL story it is still only HIS side of the story. We still will not have a frame of reference as to her thinking, feeling, etc. on the topic.
There is more here then meets the eye, it takes two to ruin a relationship just as it takes two to maintain one. So, I think we all need to back out, stop judging that which we do not know completely, and let new1 work it through with his partner only.
Yes, some of us may have been cheated on or cheated on someone before. But, each relationship is different and the 2 involved are the only qualified ones to determine the next course of action.
I understand why the question was posted, but no good will come from us validating either new1's or the other's behavior.