Unless there is a 64 bit version of the game I don't think that's possible. What he may be experiencing is not only the game, but other applications and the OS using up RAM while he plays the game.
By default a 32 bit application (even on a 64bit OS) can only use up to 2 GB of RAM. With the "large address" tweak he could have the application use up to 4GB tops. No more.