Is there a note on the casino in order to claim the winning? if there's already a note on there, the gambler should be blamed since he's careless since he didn't read it. But, if the casino didn't have notes, the casino is shady since they want to make the gambler will not able to claim his winnings.

After all, this story will make people in this forum aware to ask the casino's employee to claim the winnings in order to avoid this case happened to them. Good information @OP.