Does your Brothers machine support DDR3 @ 1333 ? ?
yes
From board manual:
"Support for DDR3 1666(O.C.)/
1333/1066 MHz memory modules"
Even if the board didn't support 1333 MHz speeds, the ram would simply throttle back to the highest speed supported by the board.
This is somewhat of a mystery, although things do not break for no reason. His brother's computer was fine, until he temporarily inserted the video card from his own computer to troubleshoot the card. When his task of troubleshooting his video card was done, and the card was removed, his brother's computer fails to boot. Hence, stuck at the "unganged mode" messege.
In a nut shell, his brother's computer was fine until he inserted the video card from his own computer.