isnt the update like every other items in the iTunes Store tied to the account it was purchased on. So that for example if you own two iPods you only have to buy it once just like music, videos, and apps work.
So while buying the update is great and you can wipe it. Who ever buys it would have to buy the update since they are on a different iTunes account. That being said I believe iTunes will see it has firmware 2.0 on it and download the right restore image.
If all else falls you can always just burn him a cd with the IPSW file on it.