this is a question of morality, lets say what if it was you who made the mistake and lost the coins? wouldn't you want your coins to be sent back to you as soon as possible?
Just do what ever to whom ever as you like others to do with your self.
Let's say it is indeed his fault for having his coins sent to the wrong person.
Putting yourself in the same situation, of course, you'd feel very bad for yourself and that person. But are you obliged to return the coins?
I don't think there is such but it would then turn to question your sense of morality as to whether you will return it or not.
