Not really Linux Mint related, so moved here.
To answer your question, what you need depends on what connector he has on his hard disk. Is it a IDE (aka ATA aka PATA) or SATA hard disk?
As you can see the SATA (modern) hard disks have a power cable and a thin data cable, and flat connectors for the cables to connect to. The IDE (old) hard disk have a power cable and a broad data cable, and pins to connect the cables to.
Once you know which it is, you go search for either an "external usb sata enclosure" or an "external usb ide enclosure". Make sure it also matches the form factor of the hard disk (which as this is an old computer will probably just be 3.5", but check! See the table here for measurements: http://en.wikipedia.org/wiki/Hard_disk_ ... rm_factors