I'm Asperger's myself. Keeping in mind, of course, that autism and Asperger's include a wide spectrum of abilities, learning music did wonders for me socially. I started on dulcimer, which is sort of an instant-gratification instrument since it can be learned relatively quickly even if you can't read music and doesn't carry the expectation of fast or intricate playing (as might a fiddle or mandolin). Not that autistic kids can't learn intricate things, but it's encouraging for beginners, especially if they feel like they're already struggling in general.

Second that one-on-one, at least initially, would probably be better. Many of us are very sensitive to peripheral noises and have an extremely hard time focusing on our own instruments if other people are playing at the same time.