I'm not an electric flyer, but I think most guys use a BEC to power the receiver and servos. It drops the voltage from what is supplied to what can be used by the receiver/servos and still supply the full power to the motor. I wouldn't try to tap off the back and use just one motor.

While your friend is correct about diodes dropping the voltage, don't do it. It just add another failure mode to the setup. And, you would need six to drop the nominal 6V to the nominal 4.8V. IE 0.2v per diode. To much complexity. It's OK for a bench setup but not what you want in the air.

Why don't you go over to the electric plane forums and pose your question there, you will find a lot more experience that I've got on the subject. Although I did stay at a Holiday Inn last night

Don