What is the ruling on an oral will made to a person other than the heirs, and do the heirs have the right to challenge the testimony of this witness?

If the heirs acknowledge the will, they must act upon it; otherwise, it must be proven. A will for a specific person involving property is proven by the testimony of two men, or one man and two women, or by one witness and the claimant's oath. As for a will made to an entity (such as the poor), there is a disagreement regarding its proof by one witness and the claimant's oath; the more probable view is the necessity of two witnesses because it is impossible to administer an oath to an entity. The majority of jurists hold that anything that is property or akin to property, such as sales, financial rights, and wills involving property, is proven by the testimony of two men or one man and two women, or by one witness and the claimant's oath, in accordance with the judgment of the Prophet, may Allah's prayers and peace be upon him.