If he's installing it to the same PC it originally came with try the phone option click here :-
"Activate Windows XP by phone
If you do not have a modem or an Internet connection, you can activate Windows XP by calling a Microsoft customer service representative. The Activation Wizard displays the toll free number that you can dial."
As long as he's not changed so many components that the system's changed beyond recognition, they'll probably give him a number.