Unless the lottery ticket was given to him for free or as a gift by someone, if not, how possible is it that one can't remember where they bought their lottery ticket from, or an event that he might have eventually won the ticket?
It might take him a little time to think, but it's not something that he can forget completely so easily. If it's a lottery ticket, he should go back to his credit card or debit card purchase history. Maybe he could be able to trace back where he made the purchase of tickets in the last few months.
It is possible to forget especially if you are hopping from one lottery shop to another. You can easily recall if you are only buying tickets occasionally. But if you are a regular lotto bettor, that would be hard if and only if, you are buying in various outlets.
This is a hard lesson for any bettor who won his ticket. Remember, if you are spending such amount to buy the ticket, then, deep inside, you have the interest of winning. So you need to bear in mind the basic requirements needed to claim such winnings.