People that have no regard for another are the kinds that do that. It never ceases to amaze me the things some people will do to avoid getting into trouble, rather than standing up and taking responsiblity for their actions.
They probably hit him, got scared and hit the gas. It seems a lot of folks these days lose their conscience when it comes to living with it or getting into trouble.

I'm sorry for your friend and I hope he recovers quickly. I hope the witneses saw something and can help find the culprit.