I'm not sure I would agree with the advise of your instructor, but this is perhaps what he wants you to learn.
The Piper Cherokee has a small amount of dihedral built into it's wings. This dihedral angle is used to "self right" the aircraft in the event of a roll.
The way it works is when an airplane starts to roll it loses altitude parallel to the slip direction:
So with a dihedral angle the wing facing the slip direction now produces more lift:
With enough time dihedral wings will right themselves, or as your instructor is telling you, you can make this faster with the rudder by turning the lower wing into the relative wind speed making it produce even more lift.
Why your instructor is preventing you from using any ailerons I'm not sure, but perhaps he has reasons you should ask him about.
This might not be the best example, but I have something personal that probably relates to what your instructor's getting at.
I ride horses quite a bit, and we have something called "no stirrup Thursday".
It's as the name sounds, when you ride on Thursday the stirrups stay in the barn.
Stirrups help you balance your weight so by spending a day without them we force ourselves to learn to balance better and ultimately stay on the horse given a situation when our feet slip out of the stirrup.
It's not unheard of that control sticks break and all your left with is the rudder pedals.
It's also not uncommon for new pilots to use the ailerons too much to correct roll and this causes the plane to roll the other direction, which they then switch the ailerons back again to try and correct it.
But I'm really just speculating at this point, you should ask your instructor directly why he is preventing you from using the ailerons.