What if he does not have a HD TV? Are you sure the AV cable itself isnt damaged? try another cable, if not i would try and use hdmi over getting a new mother board.

You can't just replace the mainboard to fix the problem, you would also have to reformat the HDD for that mainboard.   Not only that, I haven't seen any mainboards for the PS3 readily available.  That should not effect the HDMI port, but, if all you can use is the composite, then maybe take 'er apart and see if it needs some solder work on the header on the mainboard.   If not, it's going to cost whatever Sony would to repair it, and if BUYING + REPAIR >= RETAIL then it's NOT WORTH IT!   They're 399.99 new, so just do the math.   IF by chance you get a TV with an HDMI port, then it should work.