Hi Torobi,
It might be a bit dangerous to even think about repairing it, so they better find/make some backups.
Try to get a new drive and copy C: to it (use any application you like). Do NOT make it an 1:1 copy. If system fail to boot from this copy try 1:1.
On the
new drive run c: /FS:NTFS /V , then chkdsk c: /V /R
==Danger ahead, do not try (I warned you)==
You can try running convert c: /FS:NTFS /V , then chkdsk c: /V /R on their HDD.
If you can't convert maybe fixmbr could do something.
Regards
Chris