You'll find Safari's cache in your father's home directory (denoted by the ~) Here is the path:
~/Library/Caches/Safari/
Inside you'll see lots of numbered folders. Just drag them to the trash and delete them.
To repair the disk permissions, you need to boot off of the Mac OS installation CD. Before running the installer you can launch Disk Utilities from which you can then repair the disk permissions.
To boot off the CD, insert the CD, reboot the computer and hold down the 'C' key as soon as you hear the chime. Once the OS starts loading (the revolving icon) you can let go of the 'C' key.
Depending on which version of the Mac OS installation disk you are using, Disk Utilities will be located under different menus.