My first recommendation would be to test the car using the flowmeter on the test station to look for leaks with both pressure and vaccum applied. The gas cap may not leak during a vacuum test but may still be leaking under pressure.
He also needs to observe that the FTP sensor (fuel tank pressure sensor) changes with both pressure and vaccum applied. This sensor is what determines if you have a leak. The power control module (PCM) closes the vent valve and opens the purge valve, which starts pulling
a vaccum. Then the PCM closes the purge and looks to see a change in the FTP sensor. If its readings show no change, you’ll get the P0440 code.