There's three ways this is possible:
1. Your friend breeded a Pokemon, like TBB said.
2.That zoura is glit6ched and your friend got it by cheat. Or he got it from someone and they hacked it.
3.It's from a black or white 1 game when you bring Celebi to Game Freak studios in Castelina City.He probably traded it from his b/w 1 to his b/w 2.