Must the heirs pay a debt to a person who delayed claiming it after he knew of the deceased's death and attended the funeral, especially since some of the heirs have exhausted their share of the inheritance, and some inherited from a father who died after his son, the deceased?

If the claimant establishes legal proof of his right to the inheritance, or if the heirs attest to his claim, then he can reclaim that right from the heirs. His right is not forfeited by the division of the inheritance or by the heirs exhausting what they have taken. The required proof is one man and two women, or one man and the claimant's oath. The evidence for this is the hadith (definition): "The burden of proof is on the claimant, and the oath is on the one who denies."