There are two easy ways to check the catalytic convertors. Read the BARO pid during a wide open throttle acceleration. If the engine boggs down shortly after opening the throttle and the BARO starts changing rapidly, the convertors may be clogged. Also, on the 6 cylinder, look at the DPFE hoses - they may be blown apart or melted. On the 4 cylinder, take the convertor loose from the manifold (just drop it down a bit) and see if it runs better. It might help if you tell us why your friend thinks his convertors are messed up...
