The particular bank mistakenly sent the money to this man's bank account, which means this bank who sent the money is also at fault. They should be careful not to make mistake especially its a huge amount, and why they didnt verify if the receiver is the one who supposed to receive the money? Its also their negligence.

On the other side, the man spent the money in gambling. He's aware its not his money so he should coordinate to the bank if he has the conscience because its not his own. Thats the consequence of his action thus its just right for him to pay it back and face the lawsuit.