How can one help a father burdened by debt who does not wish to resort to forbidden means or usury, especially as his health condition is deteriorating?

One must strive to repay the father's debt to the best of their ability. It is forbidden to borrow with usury to repay the debt, unless the insolvent debtor fears imprisonment. One should frequently recite adhkar and supplications, such as: "O Allah, suffice me with Your lawful against Your unlawful, and enrich me with Your bounty, by Your grace, against all others besides You."