What is the ruling on preventing the non-scriptural and ill-mannered mother of the husband from seeing her son's daughter?

It is advisable to deal with such cases with moderation and balance, considering the daughter's age and her susceptibility to her grandmother's influence.

If the daughter is very young, there is no point in preventing her from seeing her grandmother; in fact, this could lead to a rift between you.

However, if she has reached the age of discernment, it is dangerous to leave her to visit the grandmother alone without supervision, for fear that she might be influenced by the grandmother's bad behaviors or her curiosity to imitate them. This could cause a conflict for her between her parents' advice and what she saw at her grandmother's. Therefore, visits should be in the presence of one of the parents to avoid any negative influence.

This does not mean a complete severing of ties with the grandmother. Rather, one should continue to communicate with her and treat her kindly, for a Muslim is a caller to good in all circumstances and never despairs of anyone's guidance.

Grandchildren are commanded to honor and maintain ties with their grandparents, even if they are not righteous, in accordance with the Almighty's saying: (But if they endeavor to make you associate with Me that of which you have no knowledge, do not obey them, but accompany them in [this] world with appropriate kindness) Luqman/15. However, honoring grandparents is not equivalent to honoring one's mother and father.