This is a case where I am inclined because the man was not to blame, well he was to blame for betting and losing what he lost, but I am in favor of something, if a person receives money, that money is already of that person, the error was on the part of the person who sent it, when the money enters a person's bank account, I see it as entering private property, no one has the right to enter without the full consent of the person who owns it,
luckily it doesn't work that way. I am sure each country has laws that protect people, companies, banks, etc... from mistakenly sending money to the wrong person. although I see your point, it would still most likely be illegal to use the money that has been mistakenly sent to your bank account. sure we can blame the sender for the error but you'd also be blamed if you withdraw and used that money knowing that the money is intended to be sent to you.