By working fine, I assume you mean the monitor goes into its test/diagnostic displays. If that is the case, most likely your roommate has hit upon more or less the correct deduction.
Two possibilites should be attempted before assuming to much more tho, one being the connector from the monitor to the computer, check for bent pins on the connector that goes into the computer. Do not worry about what appears to be a missing pin, that is normal for a SVGA connector, only look for bent pins. If you have one of those, it will have to straightened out CAREFULLY with a small metal tube as exists in some ball point pens or a small tweezer, but please be careful as they are very sensitive to over-pressure.
If a bent pin is not the cause then try reseating the video adapter card in the computer, meaning open the case ( with power off and computer unplugged ), take the screw out that holds the video card in place, pull out the video card and immediately place the card back into its slot on the motherboard. Do not try placing the card in another slot until you see if this works. put the adapter holding screw back into its former position and screw it back into place. Be careful that the adapter is properly seated in its slot, not doing so will result in NO VIDEO or even much worse conditions, such as a possible short on the motherboard, resulting in a fried motherboard. I don't think you want that. If this doesn't fix your situation then chances are as your roommate suggested the video adapter card may be bad, but there is one more possible suggestion that might just work, and that is if the video adapter is a PCI card, you may be able to place the video adapter into another PCI slot on the motherboard, above or below its present location. In a few cases that might help you ( assuming you are dealing with a PCI adapter as there is only one AGP slot on a motherboard. ). Never remove or replace components with power applied or even with the power plug still plugged into the wall socket. Newer motherboard built since 2000 will often still have some voltage applied to them from the power supply if the unit is still plugged into the wall socket.
Come back to me and let me know how it goes. Would appreciate it.