With the power off, of course:
Open up the external casing, unattach the harddrive, blow out the connector on both ends and try again.
Works for me every time.
I know this is redundant, but repeat the steps I said the first time, and if it still happens you may have to try connecting it directly to the motherboard using SATA or EIDE.
If the same result try another computer, if the same result you will at this time be looking into data recovery or if he has it backed up some other way, formatting it to use again.
Just remind him to always have his data backed up on DVDs or CD-Rs
The drive is proabably on it's way out.