I agree that reciprocal learning is a thing. The most extreme example is something I see on a semi-regular basis at work, where someone will be working on something unrelated to anything I'm expert in, come to a dead end, and stop by to bounce ideas off of me... and literally, that's all they're doing, as they have to explain the situation from the ground up. I nod and make "uh-huh" noises, and somewhere in the course of explaining it they come to a conclusion, say, "Cool! Thanks!!" and rush off to fix their problem. Ummm... you're welcome? I've been on the other side of that one-way conversation, too.
Personally, I've never advocated for individualized differentiation for my DD10, primarily because she's such an extrovert, and that approach would be so isolating. My advocacy has always been centered around the notion of getting DD placed in a new group setting with kids more similar in ability level. So if that's an option for you, that's the approach I would take, as it satisfies your concerns, as well as your school's.
If there is no such group, and you're forced into a binary proposition (no differentiation versus individualized differentiation), then my approach would be to:
1) Describe the costs/benefits of no differentiation versus individual learning.
2) Demonstrate how your DD's mastery is at a point where any improvement through teaching would be negligible.
