Is it possible to play with jus Classic&Cata ???

Sawp,

My friend is offering me to transfer one of his 80s to another account for me.

Since the character is 80 already, do i still need all expansions to be activated in order to enter a cataclysm key.?

Burning Crusade requires Classic.
Wrath of the Lich King requires Classic and Burning Crusade.
Cataclysm requires Classic, Burning Crusade and Wrath of the Lich King.

Short: yes, you still do need all expansions.