I would ask him specifically what happens.

Does the car stall? Does not stall with the radio off?

Does a fuse blow? Or no fuses blow?

And what all else is turned on when this happens? Headlights? Rear window defrost? Other heavy users of electricity?

Does this happen when stopped at a traffic signal / stop sign? (Engine/alternator running slow.)

I would install a voltmeter and see what is going on. It could be with all the other electrical loads on, the radio is the "straw" which along with a weak alternator or voltage regulator causes the car's voltage to drop too low.

There could also be a poor electrical connection and not enough power is able to get through to the inside of the vehicle. And only a limited number of electrical things could be on at the same time. (Measure voltage at battery as well as inside the car - may be drastically different!)

Or it could be that a power wire going to one of the radio components is brushing up against a sharp metal object and grounding out. Maybe just when going over bumps in the road. If that is the case, it should blow a fuse.