He did connect the light directly to the battery and still was dim, so can he assume the product is defective?
It's a consideration. If it were me, I'd connect it while the car was running to get a good ~14.7 volts, since he said the unit took 12-24V. I'm not sure to what extent he connected it directly to the battery, as that could be interpreted differently by different people; it could have simply been bypassing the switch or a circuit still through a poor ground connection.