+1 on not riding with him. I admit I don't know from mountain bikes, but I don't think the physics are any different from road bikes - and road bikes have a habit of throwing a person violently on their head.

I'm alive today (or at least rational) because of a helmet I was wearing when I stuck a pedal in a corner - as it was, I was out cold for 25 minutes. A couple of years ago I had a brain f*rt and rode off the edge of a bike path. Even though I was barely scraped and not even bruised, my head smacked the pavement pretty hard - enough that I had a headache the next day - and it would've been a significant TBI if I'd been bareheaded. It could as easily have been a log or a rock on an unpaved surface, or a paved path a MTBer would use to get to the trail.