Originally Posted by Team Plutonium
If he is planning to run the Evo system he'll run into warranty issues anyway if he encounters a SA or manager that is a dick about it. Flashing it back to stock will give him a whole other set of issues with the dealer if he is rocking a CEL.
All you have to do is clear the faults prior with the Hex tool for CAT fault codes. It takes about 15-20min of a drive cycle for it to trip the light again. No reason for the dealer to drive the car for that long unless you have a serious problem.