ما حكم تسديد دين متوفى، وما ثواب ذلك؟

Paying off your deceased friend's debt, seeking Allah's reward, is valid, sufficient, and absolves his responsibility. The deceased benefits from this without dispute, and it is among the righteous deeds whose blessings and reward continue both in this world and the Hereafter. The debt of the deceased must be paid from his estate, even if it depletes all of it, and the poverty of the heirs is not a consideration.