I'm not sure what "Extensions Off" means but you can tell the Mac to ignore the Extensions folder when the computer boots by holding the Shift key down as the system starts.
Note that Extensions add a lot of functionality to the system so it will be impractical to use the system in this "minimalist" mode for long.
But starting up with Extensions off will give you access to the hard drive and a few other things.
Then he's probably deleted the System Folder (like deleting system32 on Windows). Try booting from a boot diskette.