What should the questioner do with the money owed to a person who died, whom he had not seen for a long time, given that a friend of the deceased might not be trustworthy enough to deliver the money to his family?

If a person owes money to another, he must write a will to that effect, and not hand it over except to its owner or his agent. If it is impossible to reach the owner or if he dies, it is obligatory to find a reliable way to deliver it to his heirs. One can seek help from the owner's friends, embassies, and modern communication methods to reach them. If you do this and do not fall short in performing it, there is no sin upon you.