There are many software solutions for wiping hard drives
DBAN is one of my favorites but unfortunately if the drive does not work and or is not recognized by the computer than your options are limited and limited even further if he wants to claim the warranty since any opening or physical damage of the drive and enclosure will likely void it.
The only other option that I can think of right now would be degaussing, you might check on line to see if there is anyone in your local area offering degaussing services, in can be expensive and you may need to check concerning any actual physical impact the process might have to the drives cache chips and other components as even this process may likely void the warranty.