It's poss that this is due to his/her boot.ini file being messed up.
If they have a xp disk, tell them to boot from this and when you see the Welcome to setup press the letter R.
-They should then see a dos prompt
-Type "chkdsk /p" without the quotes and hit enter
-When that is done type "fixboot" and hit enter
"Y" and enter at the prompt
-Then type "exit" and hit enter
The system should now reboot into Windows as normal.
If that dosn't work for him/her, you can try
-Boot to the recovery console like above
-Type "chkdsk /r" then enter
-When done type "exit" and hit enter.
This takes longer, but the system should boot back into Windows.
Hope this helps.