Lots going on and you have plenty of good input. My main thought is that, first of all, it's not yours to tell S. Secondly, IMO, kids may know a lot, but they really shouldn't have to think about the gory details of their parents' relationship(s). Note that H was an adult when FIL told him. There's plenty of information that is appropriate to withhold. The challenge is figuring out the line.
Just curious whether H feels betrayed by others not having told him what FIL did?
You probably "lied" to S about Santa, too. And it probably didn't leave scars of betrayal. Not exactly the same, but still...
Personally, I am careful not to do anything to enhance anyone's awareness about their parents' affairs. Especially kids, but even adults. They don't need to know, and certainly not from me.
