Yes, that was I thought but, from the order he sent me via email, he purchased the scenery using the integrated installer, which doesn't requires to insert the serial at all (this should be needed only when purchasing using the "Buy on Web" )
Since he purchased JFK last Saturday, unless he need to reinstall Windows from scratch again, his serial should still be in the registry and active so, I still don't understand were is the problem he has.
That's why I've asked if the scenery worked in Trial. Maybe he think he should still register the scenery (because he can't see it), but if the scenery didn't worked in Trial, this doesn't have anything to do with registration, and could be an entirely different problem, like another scenery in conflict, corrupted scenery.cfg (so the installer didn't touched it), etc.
Anyway, if he really reinstalled Windows, he should simply select "Register Serial" for JFK, and type his serial. He sent me his order confirmation via email with the serial inside, so he knows it.
The only problem might arise when inserting the serial (which shouldn't be needed unless he reinstalled Windows), is that on some video card/drivers, it's best to put FS9 in Windowed mode (Alt+Enter) before selecting the Register Serial option.