Is he using the transformer?
You connect the radio to the transformer them from the transformer to the speakers.

 

It sounds like the positive or the trigger to the amp is not hooked up properly causing a surge. Make sure he has a good ground.

 

The "pop" is coming from the amp powering up. Have him check where he is getting his 12v turn on, I usually use the ignition at the radio and have not noticed any pop.

 

More than likely it's how the amp is hooked up. If power and the amp switch are hooked up to the ignition switch directly, you'll hear a pop every time. The amp's power is on when you turn the ign. sw. on. Most of the radios now a days have a wire out the back that applies a voltage to turn on an external amp (not main power). If that wire is tied to switched +12 v then the amp is on even if the radio isn't. I know, I've done it (I have a B/T amp).

Mark

 

does it pop when he cycles the radio on / off or just with the ignition?

 

Only pops when turning off the ignition switch.
Does not pop when turning off and on by the radio power button.
It does not pop when turning on the ignition switch.