It is fundamentally unimportant how the gains occurred unless the player had an inside role in the software development. Nobody is doubting that it is statistically improbable that a player won this amount. The player is not responsible for knowing if a game is flawed or not. It is also unimportant whether other players won or lost on this game. Your experience "over the years" with other games you've seen is not a credible comparison either.
So that leaves the book with two options... suspend all future payments and provide evidence of the exploitative play and demonstrate that he knowingly used a flaw in the game to win... or just pay him.