Hmmm.
If the files on drive D were set so that a normal user couldn't see them, Drive D would seem to be empty.
Unless you turn on the option to see hidden and system related files/folders, your friend wouldn't see them.
As for the tech ... well, I can only guess ...
Is it at all possible that the drive D partition is still present and just not assigned a drive letter?
Right click My Computer, choose Mange. From the Computer Mangement (Local) console, choose Storage -> Disk Management - see what volumes and disks you've got. If there is a second partition on the Disk 0, then give it a drive letter D: and reboot - See what happens.
Or is C: now the size of the entire drive? If so, I suspect you will need to do a "f'n'f" (Flush and Fill or a full reinstall). You _COULD_ try repairing things, but I doubt you'll succeed by any great amount.
If you are not getting anywhere, then burn C: to standard DVDs (so the user can simply copy files back as needed) and then do a reinstall.
You MAY have a recovery partition to allow the machine to be reset to factory default.
Both the f'n'f and factory default options should be considered the last choice. And make sure you tell your friend EXACTLY what that means.
No memory of online accounts/passwords, no documents/applications. All applications needing to be re-installed which may require license keys, etc.
Good luck.
