How can one deal with a friend who is late in repaying a debt, taking into account what is permissible and what is not permissible? Should communication with him continue, or should he be given a fixed or open grace period? Is it permissible to contact the debtor's father regarding the repayment of the debt?

The treatment of a debtor varies depending on their situation:

The affluent, defaulting debtor: It is permissible to refer their case to a judge to compel them to repay their debt, or to sell their assets to cover it, or to imprison them until they repay their debt. There is no harm in complaining about them to their father so that he may urge them to repay or repay on their behalf.

The insolvent, destitute debtor: They must be given respite, and it is not permissible to punish them. It is commendable to give charity to them by forgiving the debt or waiving a portion of it, as Allah Almighty says: "And if there is any [debtor] in hardship, then [let there be] a postponement until [a time of] ease. But if you remit [it] as charity, it is better for you, if you only knew." There is no harm in informing their father about their debt so that he may volunteer to repay it.