Most likely this is OSX Tiger (since he said it was a iMac G5)
Which password do you want to crack? User or Open Firmware? Do you have an Admin account you can log in with or a friend with an admin account?
Simple way to get in if this is your machine: Boot off the Mac OS X Install CD/DVD. Get to the screen with a menu bar at the top and select Reset Password. . . or Open Firmware Password from the top. Enter the desired password, reboot, and finish your beer.
To get the OF password, google FWSucker by mSec. Download, run, and finish your beer.
To actually "crack" a user password use "sudo nidump passwd / > somefile. passwd". You'll have to type in your admin password for the sudo command or trick a friend to type in theirs (easier when using a shell script dressed up in AppleScript/Automator). Then take the output file and run a commonly used password cracker (maybe one that rhymes with Mom the Stripper) on the file using your own computer. Finish your beer while the program cracks the password (okay several beers -- it takes a while.)
PS. This is for informational purposes only. I "assume" the individual has simply forgotten his "own" password to his "own" computer as he/she is "pissed" about not getting into it. There would obviously be no frustration or shame in not being able to get into someone else's computer.
