Cheating is the only way to get multiple master balls.
To be fair, you can get multiple master balls by having multiple game versions too. Heck, I bought a second copy of Emerald just so that I can run through the game, transfer anything worthwhile off it and then start a new game. It's given me a sizeable colection of Master balls, TMs 1 to 50 and so on. Though I doubt he'd want to or could do this.