You're assuming he's using Tiger (which might not be the case), but yes, loading from the disc and doing an Erase and Install or loading Disk Utility from the menu and formatting the drive will do the trick.
However, OS 9 doesn't take that much space on its own. An install is usually about the amount barhar mentioned at most just for the operating system (give or take). It's possible that there are other Classic applications that are taking up the rest of that space, so formatting it would be good. You can try installing OS 9 back on it so that you can use Classic, but that's up to you. If you don't have any applications that used to run on OS 9 (and I don't think you would since you have only used Windows previously), then you don't need OS 9 at all.
BTW, which model iMac do you have? Check in System Preferences for the specs. Depending on the iMac, you might need to install OS X on the first 8 GB of the hard drive (meaning you have to create an 8 GB partition, or 7.5 GB to be exact, just for the operating system and the rest can be partitioned for applications and docs).