hmm, i don't think it is possible to buy FCPX on discs. Are you sure he's not talking about FCP7? (which you can legally buy from him)
Regarding FCPX, there is no mechanism available to disconnect his account from the software, and reconnect it to another one. Same thing for anything you purchase via iTunes -- it's not like you can transfer music/videos to another account.
The only chance you might have is for him to call App Store support and ask. Another possibility is for him to ask for a refund, in a manner that's been well documented in the early days of this forum. Then you purchase it yourself.
Now that apple is fully embracing digital distribution, maybe things will change with regards to what you can and can't do regarding "ownership" of a digitally purchased software license.