OT, I honestly do appreciate your devil's-advocate stance most of the time, but I think you're reading way too much into the grandparents' actions. You and we don't know the situation and perhaps Donna might elaborate more on it. It is highly conceivable that the grandparents have done the best they can to accept their son's transgressions, extended every courtesy possible, but the son (Donna's ex) might very well be insisting on being obstinate and refusing to repent of his offenses. He could very well be violating the moral boundaries they have established through demands that they accept his new family without a shred of contrition on his part or any acknowledgment of his wrongs. That might very well be the situation, that his parents would love to forgive and forget but the son insists that they act the ignorant fools and forgo any sort of boundaries or value system they have tried to uphold -- it's just as plausible a rationale as the one you seem to be asserting.
It would be entirely different if the ex were approaching his mother and father telling them he knows he has screwed things up royally, going against the values they tried to instill in him, that he truly hurt Donna and wrecked their family, but now he loves this other person and they have forged a new family, flawed as it is -- and for him to appeal to his parents' mercy and to forgive him of his wrong-doings. If the parents weren't able to accept the prodigal after that, THEN you might have an argument. But you can't say one way or the other given what we know.
But more to the point, in these forums, we advocate over and over again, ad nauseum, that we cannot control another person's behaviors. We can only control ourselves, and we try not to concern ourselves with things beyond our own control and certainly beyond our concern. The R between Donna's ex and his parents are none of our business and not something Dona need concern herself with directly. If it is having a deleterious effect on her children, yes, that's unfortunate, and Donna should help them, the children, from her end, but she's not obligated to do anything more. It's out of her hands. Deal with the kids, but move on. It's what everyone in DB has been trying to beat into my head for months and months now.
Donna, if I have mistaken the situation, please feel free to correct me.