why should he need a flash cart?
he wants "a device to back up GBC saved games to PC from a GB advance console."
you need a reader/writer. the gbc devices were usually standalone devices, not needing a console (this changed during the gba era, when the gba was used as the reader/writer).
with the reader/writer you can dump your own games and saves. also, (not confirmed!) you should be able to rewrite your savegames back onto the real games.