Ultimately, assuming that everything is as it seems, bendavis should give the coins back. I know that no authority is going to make him do that however if the case were to go into a British court (only because that is the only country that I have knowledge of) then it would be pretty clear that bendavis should give the money back, possibly with a small amount of compensation to bendavis from the exchange for having wasted his time with their mistake.

Seriously though, how could anyone honestly think that just because someone makes a mistake that it gives you the right to keep money, that you haven't done anything to receive, as a result.