This is probably due to System Restore, make sure his computer is set to use a nominal amount of space for System Restore (say %10).
If you want to get into the nitty gritty and learn about how much space Volume Shadow Copy is using, launch an Elevated Command Prompt and check out the following:
vssadmin list shadowstorage (shows storage usage)
vssadmin Resize ShadowStorage /For=C: /On=C: /Maxsize=10GB (sets max size of Shadow Copy Storage to 10GB)