Bizman, what if he's got a HardDrive that he has stuff on?
Right clicking the Start icon brings up a variety of shortcuts, one of which is Disk Management. It should show any hard disks and partitions you have installed. You probably will find your missing 350 GB partition there, all you have to do is to format it.
Possibly go into Disk Management and if he can see it check to make it active, then reboot so it is registered in his MBR correctly?
if this is the case and it's not seeing it at boot up, but is visible as inactive in Disk Management and it does have stuff using a different OS version ie: WindowsXP, Windows 7 all versions, Windows 8/8.1 all versions
Windows 10 will / should automatically install KB updates to that HD and work, if I remember correctly after a reboot
Agree on the formatting 2nd partition of the Windows 10 HD with 2 partitions
Edit: should have included that MS has had issues with using a newer OS version and trying to install an old HD with older OS on it... MS preference is to go through this route in chronological order
Been a while since I last messed with all that stuff
Maybe and/or Hope this helps... Best of luck