I think you should leave the decision to your nephew's teacher, just don't get involve in something like that.
This is a difficult thing to instill in young students - Learn many easy pieces because it will make the harder pieces easier.
So you can spend the rest of your life playing "difficult" pieces for yourself and at the end of 10 years you have 40 really hard pieces under your belt, or you can spend 10 years learning easier pieces, so you have 200 pieces under your belt and then turn your attention to these harder 40 pieces and eat through them in less than 2 years beacuse of the application of knowledge instead of brute force.
.... I certainly see the logic from the standpoint of learning more easier pieces quickly vs fewer hard pieces within a certain period. What I would have a hard time explaining to my nephew is how the application of knowledge from those easy pieces would speed the learning of a tougher work.