The answer
Your friend is probably right. The revs should not drop greatly because the engine would compensate for any drops in revs. The extra draw of current from the amplifier does not sap power from the engine, just puts more strain on the alternator and makes the engine work a little harder. As the revs drop below the normal idle, the ECU lets a little more air and a bit more fuel into the engine, bringing it back up
As for the rev counter, with the Nissan, like many modern cars, the rev counter is not controlled by the engine. The ECU uses the sensors around the system, i.e. crankshaft sensor and camshaft sensor to determine the speed of the engine. The ECU then sends a voltage to the rev counter and displays the value on the gauge. If the ECU is sapped of power because of the extra drain on the battery, it will give false readings.
If your car's idle dropped to 300 it would stall.
Ed