One thing worth mentioning is if he intends on using both for playback that is fine, nothing needs to be done. But if he is going to get rid of the Powerbook, or otherwise mess with the OS, I'd strongly recommend deauthorizing that computer from being able to play iTunes stuff. If you don't, that configuration is authorized to play whether or not it still exists. Apple gives you 5 computers you can have authorized.
This may seem like no big deal, but, lets say something breaks like the HD in the Windows machine (or the powerbook, doesn't matter), and you reinstall. Now you want iTunes to play what you bought, you have to re-authorize and that is an additional authorization. So now you are up to 3 (4 if both hds magically died).
I admit, its not a problem for 99% of people. And you can deauthorize all computers (and reauthorize individually), but I think you can only do that once a year.
So basically, what I'm saying is, if you can deauthorize a computer that you don't plan on using for iTunes anymore, then it is better to do it than not.
