This is said tongue in cheek yes?
Didn't throw you £100 no? I'd have been very pissed off. He's doing the Lotto so he is already being taxed for being an idiot. He should have to pay the higher rate of not even winning if he gets the right numbers, for not checking his ticket properly.
There was a story in Ireland recently where a man bought a lotto ticket in a shop, but didn't pick it up when he was leaving. The gent behind the counter kept the ticket for him and it won €350,000. The next day he called the man and told him to come in to collect his ticket.
How much of a cut would you give the gent? Would you have given it to him at all? It sounds like you would definitely get away with it