Hmm. This is not easy, but a few things do come to mind:
There's a peculiar little book called The Wonder that is Sanskrit, pub. by the Sri Aurobindo Society,
that has a chapter on Sanskrit grammar that, despite some flaws, does introduce some majpr
features in terms accessible to non-Indologists.
Frits Staal's article, "Euclid and Panini," reprinted in his book Universals, is a bit technical,
but very good. The comparative dimension is either illuminating or obfuscating, depending on
Bimal Matilal's book The Word and the World, is really on philosophy of language, but includes
much on the grammatical traditions. It's a gem beyond price in my view - one of Matilal's greatest
successes. I'd suggest copying the first chapter or two for your friend and, if he goes for it, he'll want
to work through the rest.
Ecole Pratique des Hautes Etudes
Numata Visiting Professor of Buddhist Studies,
The University of Chicago