First thing your friend will need to do is have the EGR problem repaired so that the check engine light will turn off. P0401 EGR insufficient flow indicates, more than likely, the EGR valve or plumbing is plugged up with carbon deposits, not allowing adequate exhaust to cycle through the EGR valve and into the exhaust manifold.
Once the trouble code P0401 is repaired, the system's ECU should be reset and the Diamante driven for at least 200 miles (in both highway and city traffic conditions) in order to get the emission monitors ready for a smog check retest. If during the drive the check engine light once again illuminates, futher diagnosis will be required. It is possible that P0421 re-triggers and the catalytic converter actually needs replacement. Intrusive testing should be conducted before replacing the CAT.
