I doubt that the hard drive would prevent the screen from displaying anything. When you boot without a harddrive it will still show something on the monitor, usually something like "Boot device not found, please insert boot device" etc..
So look into resetting the CMOS. Different mobos will use different methods so research that model to find the exact location / method for your buddy's computer.
Unplug power cable, then hit power button one or two times to clear capacitor's residual charges.
Either remove the BIOS Battery, reset CMOS jumper, or both, unplug harddrive and then plug power in and boot computer for a few seconds. Shut down, unplug power and hit power button to cycle juice out of the capacitors, then reconnect everything and boot up. BIOS should be in default and monitor may work then.