Hi Stephen,
I see from your post that your friend's iMac is prompting him to enter a password of some kind. The only password I can think of for a system would be the administrator password, such as the one that's required to install downloaded applications. In order to reset that password you will need to boot the computer into recovery mode by following the directions below.
1. Restart your Mac and hold down the Command + R keys at the same time right after you hear the chime
2. When the boot completes you will see the OS X Utilities window. Do not select anything in this window.
3. In the menu bar select Utilities > Terminal
4. Type resetpassword (no space between the words) and press Return on your keyboard
5. In the new window that opens, select your Macintosh HD and enter your new password (and hint if you choose)
6. Select Save
7. Select OS X Utilities > Quit OS X Utilities from the menu bar
8. This will prompt you to restart your Mac
9. After restart, enter your new password to log in
Have a great weekend.