As far as I can tell, the only compilation with Dragon Spirit on it is the PlayStation Namco Museum games. I'm afraid that your friend is probably just mistaken.
Though she could be talking of Dragon Saber, the sequel to Spirit. The game was designed on System 16 hardware, and if I recall, that hardware was designed by Sega to allow it's games to be easily ported to the Genesis. As far as I can tell, there's no version of the game for Genesis or Mega Drive.
You could always get the TG-16/PC Engine version.