There is everything wrong with spending a money that was accidentally sent to you. Worst still, the money is this huge and should raise a lot of concerns in the mind of whom got the credit. His actions of not making enquiries and going straight to spending the outrageous amount and in gambling even tells he's got plans of his own and was ready to face the consequences. In instances like this, you get to contact your family and friends to be sure without stating the sum and then you proceed to verify from your bank, if they could supposedly contact the sender.

Fir all we know, it could be a scam deal for which, they want to frame someone or divert attention and it would be difficult explaining your innocence haven't spent the money already.

Perhaps, this was he trying to make some profit his own off the money before making further verifications and it went south. Unfortunately, forgiveness isn't the way to go about this and his got to return what he spent.