I understood that Donna's X was trying to tell their son that his A did not cause the D. My point was that he is really probably not lying at all. It is probably better for all involved if they accept the truth of that.
Now, if Donna's X was trying to tell their son that there was no A, well, that is a flat out lie. Not sure what I'd want to do about that, I'd probably talk to a C.
As for using an X-spouse to moralize and teach kids lessons, I think there are plenty of other cases that can be used to teach kids morals. A child needs parents that they can love without feeling like they are betraying the other parent. If one parent paints the other parent as the evil-doer, that becomes pretty darn hard.
I think probably the best way to handle what info is shared with kids is this: Imagine you are reconciled and all is well. Then ask, what info would you want to have been shared with the kids and how would you want to have shared it? If you share that info in that manner, I think you'd probably be doing the best you could for your kids.
