Yes, you can do as MISRy said. Use the disk management tool shrink/expand a volume.
Would I do that - No.
There are very good reasons for using two partitions.
"C" drive Only for Operating system and Programs. For most 120 gigs works, 200 should be more than enough.
"D" drive should be for all the files that he creates. IE move all the autocad drawings he has created, photo, Movies, ie to the D drive. Will need to re-link but that is easy.
Now for the reason, you did not indicate operating system so I'll use Win 7.
The C drive can be backed up very simply by using win 7 back up that will create an image file of his boot drive. This only needs to be redone when when new programs are installed. In some cases this could be on a semiannual basis. Note: if you lose your boot drive (And DID NOT Back-up) you can recover by simply re-installing the operating system and programs. Windows 7 has an outstanding back-up program to create an image and for restoring that image under the control panel (not avail in Vista and XP, but 3rd party programs are available).
The Data that a user creates CAN NOT be recreated if the HDD goes belly up. With all his data/files on the D drive, this could be backed up on a daily/weekly basis - and MUCH similer if all on D drive.
My advise - Leave as is and teach good "filing" habits. Operating system and Program
in Top drawer (C). Files that he creates, downloads, and other data that he puts on goes in the 2nd drawer (D) and a folder for each group.