It was surprising because if he could figure out why the money was in his account, he certainly wouldn't be so stupid, even if he had a gambling habit or problem. Well, this might be a huge temptation for him because he sees big money coming into his account and thinks he won a bonus or lucky draw.
Having a gambling problem automatically made him vulnerable to tempation. I bet that guys gambling urge is too strong that he dismissed any rational thinking that he don't own the money and continue to indulge himself in gambling.
But everything has happened and I hope he doesn't repeat that mistake. Otherwise, he would face even greater consequences than this.
I don't think he will have enough money to repeat that mistakes again. Remember he promised to pay and I think it will take more than a lifetime to return the money.