You can’t just put the drive from one computer into another and have it boot from that because of the differences in motherboards, chip sets, drivers, etc. unless the systems are totally identical. You could put it in and access the data on it as a secondary drive, though.
Hmmm, I don’t think it would be as difficult as you think. I just did a clean Windows install on a Dell, and it boots up fine without the chipset, video, etc drivers installed. You install all of those from the resource CD after you boot to Windows. Until then, it operated on default drivers included in Windows. I’d imagine it would be a similar operation with the old HD, but he might have to boot the old drive in safe mode the first time in order to get the default drivers to function instead of the ones from his old system.
Of course, he will need to make sure the drivers on his resource CD include the XP drivers. Dell resource CD’s usually have drivers for multiple similar models included on the same disk, so they might include drivers for legacy OS’s as well, but I can’t say for certain.