He must be a very rich person who have 1500 Bitcoins with him and that too he risked all of them in gambling. He was lucky enough to won 51BTC but the risk involved here was too much. He could have lost all this amount if he had lost the game. 
You don't need to have 1500 BTC, It's just an accumulation of what he has ever wagered in life time. Anyway, no wonder if he won that much. It seems he is a true gambler who just got luck to win this huge prize (sorry to say luck as I believe there is luck more than effort/strategy).