Chances are he didn't. Verified dumps are all over the internet. However, a sufficiently motivated individual can create cartridge dumpers with hardware like

Chances are he didn't. Verified dumps are all over the internet. However, a sufficiently motivated individual can create cartridge dumpers with hardware like Arduino and a broken Game Boy (to harvest the socket for convenience). The cartridges for most/all game systems are widely understood by this point thanks to the efforts of reverse engineers. Of course there are also pre-built dumpers available through various sources. They are not unique to the present-day. Even when the SNES and Game Boy were still selling on shelves, there were devices created specifically for copying cartridges to alternative storage such as floppy disk or custom cartridges.