If this were in my area (So Cal), I'd say they may have been an illegal alien who didn't want to get deported. Or they may not have insurance. Could be someone running from the law. Could be an immature teenager that freaked out and reacted before thinking it through.

Any way you look at it, it's a very sad thing to hit someone like that and just leave. I'm so glad your friend is okay. Best wishes for a speedy recovery.