Have him use a hand-held tester in the outlet, with the breaker on and then off. This will tell him if the hot & neutral are reversed. Also, just a guess, but perhaps there is an error in the wiring of the branch circuit such that the hots from two different breakers are spliced together somewhere? If he has a handheld voltmeter he can check for voltage at the output of the circuit breaker with the breaker in the off position.
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