Is it permissible to give charity (sadaqah) or Zakat to a non-Muslim friend to pay off his interest-based debt, or to give him a benevolent loan (qard hasan) to pay off this debt?

Zakat, which is obligatory, should not be given to a non-Muslim. However, voluntary charity (sadaqat) may be given to him, although Muslims have a greater right to it. As for lending money to a non-Muslim, it is permissible. If the loan or charity will be used to repay an interest-based loan or to encourage interest-based borrowing, it is better to direct the assistance to another person or area, unless the aim is to invite him to Islam and show him its virtues.