Have to ask: Is the monitor plugged into the motherboard video port or the GPU video port?
If you do correctly have your video cable plugged into the GPU then you may need to reset your bios as per the instructions in your motherboard manual.
The way you described your situation earlier I thought you had a motherboard with no video outputs, now knowing you have an FM2 board it could be set to onboard video output.
On a side note, your cousin should upgrade that power supply IMMEDIATLEY. While granted a GT 420 is not pulling a very heavy load, it is still an extra load and logisys power supplies are cheap junk that die easily, and when they die they take out the rest of the computer with it.
Rule of thumb: never go cheap on the part that can destroy everything else.