What UmbraLux said, more or less.
Consider that 100 generations for normal mortals is about, what, 2000-2500 years? In the last 2000 years, humanity has changed in only small ways. We've gotten a bit taller, for example, and have gained immunities to some diseases. But nothing that would make them physically 'unrecognizable'. Their mindset would likely be extremely different, of course.
It's harder to say what would happen in your scenario, though. What's 100 generations to a semi-immortal? It could be as little as 2000 years, or it could be 100000 or more. That sort of timescale brings the possibility that the isolated group might have come from a branch of humanity that has dies out in the mundane world. Neanderthals, for example, *would* look markedly different.
And, of course, living in the Nevernever is also a wildcard. I could easily see the possibility of significant 'genetic drift' over the course of a human's lifespan, let alone a hundred generations.
As a final variable, what effect would wizard's constitution have on such individuals? If it has the effect of continually maintaining them in their original form (effectively fighting off any changes the Nevernever might try to impose), then this would tend to reduce the changes that occur over time.
Bottom line is *anything* could happen, and it could happen completely differently to two different isolated groups. So as far as your debate is concerned, you're probably both right.
