Receiving such a huge amount of money which is unexpected is too alarming. He should double-check it in the first place or question it but it's his fault for using it to gamble intentionally.
He really meant to spend it and he might actually have an idea that it was sent to him by mistake so he gambled it right away.
Mistakes like that happen all the time, it is very common to see stories of people receiving millions of dollars into their accounts out of nowhere, and most of the time what you see is that the people inform of the error and then it is corrected quickly by the bank without any issue, the difference is that this person saw that amount of money in his account and did not asked any questions and immediately assumed that money to be his, and now he is paying the consequences of such a dumb mistake.