I have never used three channel to teach with but I did learn {kinda sorta} to fly with a little two channel Cub, it had elevator and rudder, both on one stick/Gimbel. After flying it a few times {never soloed with it} I went over to a giant Sr. Telemaster that had all controls, including flaps. That was my real trainer.
As an instructor I have never removed the ailerons on a plane and don't understand why I would do that to a student?
I just got in a set of plans for the Old Timmer Satyr that is supposed to be a three channel converted free flight plane from 1942. I will be building two of them, one for myself and one as a gift to a friend. Both will be electric powered and three channel. Both will be set up with the elevator and rudder on the right stick.
I love flying three channels, it's very relaxing but it is nothing I would ever teach with. I guess I could though, a lot of the glider/trainers are three channel.
I wouldn't say your friend is completely wrong, it is just something we don't do these days. We have the buddy box to train with and a student can't do anything to the plane I can't correct but there are a lot of people flying today that learned with a Gentle Lady or Butterfly three channel. Even more that learned on the Kaydet that John mentioned.