Getting arma 2 OA with just the CD key

Hello, My friend wants to download arma 2 OA but doesnt have any money for it. I have a CD key he can use, Would downloading the demo and then just replacing the key work?

It Might do someone on the forums Said they had done that and it works for them if not then he will just have to buy Operation arrowhead and just download arma 2 free.

Why not just give him a copy of the game, and buy a key off here? Or just torrent it?

If he is using steam go to activate a product on steam in the games menu and enter the key.....