most likely yes, but wouldn't the os be already saved in your hdd ?
if you have the version of XP that the key is intended for, then yes it will work.
you just might get lucky and not have to do a reinstall of the OS. or perhaps be able to do a repair.
No matter what, he will have to do a full reinstall!
why is that?
Drivers are incorrect. It will cause an unstable OS and possibly just give you a BSOD all the time.