Would you let someone use your car to test a suspected electrical fault? I would be very wary as modern car electronics can be fussy.
Yes, other than it would be me using it. At the worse it will cost me one or more of the spare blade fuses from my in car stock. Though even that is most unlikely if the unit has a history of working over the last two years.
But long before I bother to move my vehicle I would check his rear socket has positive 12 v on pin 9 relative to its earth pin 13, with a muti meter, again kit carried on board. That I must have used sorting way more other peoples problems out than my own.
Also in the kit is my car recharging adaptor, a plug wired to the appropriate pins, so even fathoming out which to stick the meter's probes on is side stepped.
All said, on info given I sense he is being fobbed off by the dealer, far from something unknown.