If he was playing online, randomizer is a version of the game on an emulator (so automatically a ROM file) that completely randomizes:
- Starter Pokemon & their items
- Wild Pokemon, including event Pokemon
- All other in-game Pokemon, including trades and prizes
- Trainer Pokemon
- Trainer titles (like 'Lass' or 'Pokemaniac')
- Items found on the map, including hidden items
- TM attacks
- Pokemon-TM compatability
- Pokemon themselves, including types, stats, palettes, abilities,
wild hold items, and movesets
- Redefining certain evolutions so trading is not required
- Changing some core game things, like stacking TMs in groups of 90,
starting with the National Pokedex, or modifying store stocks.
Hope I helped!