Down shift before the corner/curve so you can gently accelerate thru and out of the corner. Never coast with the car in neutral or the clutch in. I think it might be against some state laws as to having control.
You can rev the engine while the clutch is in to bring the rpm up to a level to match the gear you want to down shift to. This is just a practice thing that you learn from listening to the engine, watching the tach vs speed and trying different things.
Big plus 1 on that! Think of it as giving up one of the main control parameters you have. It also effects Steering and you can even get some useful engine braking to fine tune your speed in different circumstances.
Your friend is dispensing bad advice, dangerous really.
If they are available nearby, you and your friend could pick up some useful tips and have a great time at a track-day / race driving school. There you will learn the balance of steering , braking and throttle control.
You can't learn that on the internet!