Yes and no. It is definatly illegal to upload that, however it is not illegal to download it and install if you have a valid serial number. Your best best would be to find the OEM xp install for his brand system. The OEM are pre activated, and would match what he should have on his system. But it is shady, but considering that they don't even ship new pc's with these disc anymore, it may be your only choice.
Also you should know that even if you have a valid serial number, it will not activate will all xp install discs, even if it matches the version. I once tried my father in laws pc. I made him a new xp disc, xp home with n-lite. I used my dell xp disc to make it and put in his SN. No luck with that, the damn thing would not active. So a made another disc with nlite using his disc, and that worked just fine- both disc where xp home.
If he has an xp home sticker on the side of the pc and you install xp, I think your fine, at least in my book of ethics. Now if you put pro on, that is different.