If he's still got hid old XP disk, just put that in and restart the computer. It 'should' boot from the CD and he can reinstall XP. If it doesn't boot, you will need to change the boot options in the BIOS. Soon after the PC is switched on, you'll get a beep or two and an option to press F12, F2 or Del. In there should list the boot order. Make sure CD is the first one.
As my previous post demonstrates, I don't have much sympathy with the guy but the above should get him back on the road to recovery.