The odds of being in fatal crash are quite low, but that's hardly the only situation where it could come up.
In California, the minimum insurance coverage is just $15,000 for personal injury and $5,000 for property damage. You could be involved in a minor crash and easily exceed those minimums.
And again, even if you're in a fender bender that is fully covered by insurance, that could impact your friends insurance rates for the next 5 years.
I'd just stress the larger point that it is possible to do this, but you and your friend need to know exactly what you are getting yourself into and the risks and responsibilities of this idea.