Not knowing what 'going bad' means in this circumstance I can only answer generally. I'm assuming it's a hard failure and you've already tried to boot off of the install CD and run Disk Doctor (in the Utilities folder). I'm also assuming you don't have any other Mac drive utilities (like Diskwarrior or TechTool) around. You also didn't say what kind of G4 (Cube, Tower or PowerBook) so I'll just cover tower or PowerBook.

The G4 towers have EIDE as the default but are also available with the option of Ultra160 SCSI, so try to figure out which your friend has. If the machine isn't completely dead you may be able to boot off the OS install CD (hold down the C key while restarting) and run Drive Setup (it's in the Utilities folder on the CD) to discover what kind of drive(s) you've got. Otherwise, you've got to open it up and poke around. (Fortunately, the case of the tower is very easy to open and the guts are readily accessible.

If it's a PowerBook and you need a whole new drive, I can refer you to this article on PB drive replacement options. ([link|http://www.lowendmac.com/misc/20000104.html|http://www.lowendma...0000104.html])

Once you replace the drive, if necessary, you should be able to use Drive setup to partition and initialize the new drive.

That's what I've got for now until more details are forthcoming.

The way that *always* works (when the C key thing doesn't) is

<Shift>-<Cmd>-<Option>-<Del>

Not as intuitive, but usually works on older Macs (say, from the Power Computing days).

Another way, if you can boot to your hard drive and are running OS 9 or better, is to use the Startup Disk control panel.

Of course, I'm also assuming you're not being sarcastic....