I know in the BIOS of some of the older Dells, you could actual enable/disable specific SATA connections. Just because the power and comms cords are plugged in to the motherboard, it doesn't mean it'll actually read the drive.
I can't remember the exact placement in the menu off the top of my head, but there should be a list of SATA ports (0,1,2,3, etc.) in the BIOS and the ones for his working HDD and CD/DVD drive will be enabled. I would suggest turning them all on so he can see if the drive is recognised, and it'll tell him what's connected once the port is "made live". After that, he can disable the unused ports.
"An ounce of prevention is worth a pound of cure" ~ Ben Franklin