O-scope does not show distortion. So either way most likely he will have to buy an oscope or a dd-1, on top of a quality True RMS AC Current clamp meter, and a True RMS Digital multimeter. So a easy $600 in test equitement just to test something that a reputability company already knows and told you, and backed by many users real world clamp tests.
But in the end, if you buy a legit amp, you can trust its ratings. no need to go thru all this to see what power your amp makes on a fixed resistance.. It will be very simular to the manufactures tests and ratings (as long as your not using sony, pyle jensen, pyrmid, power akoustik, ssl, etc, etc).