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.
Of course, I agree with you. My advice was based on the underlying problem and knowledge of my SILs mother. My daughter is stubborn - just like me, and she can hold a grudge. If I felt my grand daughters were in danger such as you noted, I would have told her differently.
Thanks,