It wouldn't hurt to try. The software itself isn't tied to any individual key, so you can freely get it at
http://www.guildwars.com/downloads/clientdownload.html (ignore all the beta weekend stuff, it's not relevant anymore). When you create an account it may very well be possible to buy a cd-key online.
I'm sorry I can't offer you a real answer regarding this. It never hurts to explore, and you're not doing anything bad by downloading the software.
Let us know what you find out
