In the first place is it his money?, why won't he be at fault ,If he was a good citizen the rightful thing he should have done is to return the money, the money that was sent wrongly to his account, he went futher and used the money for gambling. maybe he was aiming at winning a jackpot and then pay back the money. So in all and sundry he's at fault and not the sender.
the gambler was indeed at fault because he had used money that was not meant for him. but on the other hand the sender also has a little mistake sending large amounts of money but is not careful in writing the address to be sent.
in the end the gambler also admitted his mistake and intended to return the money he had used to gamble.
so the gambler is not completely wrong