My daughter and her husband’s mother have some deep problems between them. She’d once said I don’t want my kids around her. What I told her is that regardless of how she feels about her husband’s mother, she shouldn’t keep the children from her, at least in cases of holidays, birthdays, the occasional weekend stay, etc.
She can ‘deprogram’ the kids when they get home if she has to. Otherwise, it would do nothing but create more problems with the grandmother, and even worse problems with her husband.
I think that the best course of action is highly dependent on the circumstances.
If the grandparents behavior is so outrageous that it becomes a matter of the children’s psychological or physical safety then it is not right to rely on “deprogramming” just to make everyone happy. For example a child witnessing an alcoholic tirade is not easily deprogrammed. Once innocence is lost it can not be recovered. Also undermining parental teachings regarding matters of morality could be confusing to children.
On the other hand if the nature of the disagreements are more personal in nature then your approach would work well. For example if MIL really wanted son to marry someone else.
I am not making a judgment on the advice you gave to your daughter just pointing out that this doesn’t work in every case.