A CD-Key is only used once, on the WoW website, to unlock an account. Once it has been used, it cannot be used again. You do not login to the game with a CD-Key. His CD-Key will do nothing for you. The free trial thing is different though. I believe if you do that you can continue with a real account, but you have to pay.
The trial thing requires activation with a $45 first month (from a box) when it's over, and you can't trade with others untill you pay for your account.
Did you not read where I said "you must pay $45 for your first month"? That means exactly what I said; you must go to the store and buy a $45 game box in order to play your first month.