Is the husband obligated to spend on his extravagant and wasteful father who incurs debt despite his abundant wealth? Is the father considered in need in this case? And do the spouses incur sin if they do not spend on him?

The majority of scholars hold that a father does not have the right to take his son's money without his consent if the son is in sufficient means. This differs from the Hanbalis, who believe he may do so even if there is no need, provided he does not harm the son's wealth or give it to another child. Accordingly, your husband's father does not have the right to take his son's money if it would harm him or if the son needs it to support himself and his family; for the maintenance of a wife and children takes precedence. If there is money left over after spending, the son may refuse to give it to his father if there is no need, and this is not considered an act of disobedience, but he must still be dutiful and benevolent towards him.