Problem child/running rich

im a copier tech trying help a friend :) 2003 1.8L Corolla... LTFT is negative 14, P0420 and misfire codes... symptoms: rough idle, plugs keep fouling, cat ruined again replaced injectors, cat and O2 sensor, new intake gasket, new fuel regulator, new coils and 3 sets plugs... cleaned MAF, airbox to Tbody, and the 2nd O2 sensor data now mirrors the first O2 .. i suppose the cat ruined again. This is all i know from my buddy whos working on this car for widowed Mom with 3 going up soon see if i can diagnose.. heres my wonderings: high neg LTFT is car trying to lean out...could the cold start injector be stuck on? could the EVAP purge valve be stuck richening the mix? Another question: a vacuum or exhaust leak wouldnt cause high -LTFT right? that would be extra oxygen arriving at the O2 sensor thus the ECU would do a positive trim correct? I will try get more exact PID info..