It's probably just a save file hack. Either change the value in RAM on his highly modified 3DS, then save, or else use that 3DS to extract the save in decrypted form, edit it, then replace it and have the game re-encrypt it. Either that or he figured out save file decryption/encryption.
So it is not the first option. Maybe he already knows how to [de]encrypt the save file.Oh, and by the way... did I mention this is with an unmodified 3DS? ;-)