What is the ruling of Islamic law regarding a debtor husband who gives money to his family for non-essential matters, and borrows from his wife to pay off his debts?

First: One must avoid terms such as "the opinion of religion" and "the opinion of Sharia," because an opinion can be right or wrong. However, what Allah has decreed in His Book and the Sunnah of His Messenger (peace be upon him) is referred to as "the ruling of Allah" or "His decree." If a ruling is the result of ijtihad (independent reasoning), then it is called "the opinion of the mujtahid" or "the scholar."

Second: Children must spend on their parents if they are able and the parents are in need. This obligation is established by the Quran, the Sunnah, and scholarly consensus. If the parents are in need of essential sustenance, then there is a great reward for the child, even if it requires taking a loan. However, if they are not in need and the child spends on them for luxuries, then he should not take a loan for this, because the matter of debt is serious. If he has surplus money, he may spend it to provide them with more. But if he does so with his wife's money or someone else's, he is wronging himself. He should gently apologize to them if he cannot fulfill their requests, and promise them to give if he obtains surplus money, in accordance with the verse: (And if you turn away from them, seeking mercy from your Lord, which you expect, then speak to them a gentle word) Al-Isra/28.