About Guild Wars Key Thingy.

Yeah, its been a looooooong time since i have played guild wars.
I bought factions first, and i played guild wars for a while. It got boring
because I had crap builds because i only had skills from factions.
Anyways, more to the point, I got Nightfall, from a friend, but I do not
have the code that comes with it. Do I still need one from Nightfall?
Or is it okay because i have an account already in another guild wars game?

you need key from it.. ..

So you have two accounts that are not linked. Even if your friend gave you the key, the account was already created and they can not be linked now.