If it was a graphic card problem you would hear the os jingle, the fan would run and you may get a no signal error message on the screen, even as you say with an onboard graphic card.
The motherboard supports all of your CPU' components as all parts are connected to it even the HD and other hardware.
Your PSU test was successful, I can't see anyting else but the mummy board.
When was the last time your friend used the machine? What happened in between? Was it knocked sometime in storage or move? Was there a power surge? I am just guessing personafying Sherlock Holmes.
Dell is not the most reliable. How old is this machine?