You don't have to be going that fast to get a concussion. I was getting off the lift one time and rammed into a person that was just standing on the off ramp. She saw me come and pushed me back. We both fell but I fell while flipped back and landed on the back of my head with a resounding thud. I was dizzy for a good 20 seconds.
Good thing I was wearing my helmet. It was only later that I realized the fall broke the suspension straps/frame inside my helmet. If I were not wearing a helmet I would have gotten a minor concussion, if not worse.
Speed has nothing to do with it, it's the impact. I didn't assume speed was involved, especially since he said his friend was new to riding. I'm really just curious what happened that caused his friend to hit his head. I often feel that I unconsciously do everything possible to protect my head (and ride with more caution) since I don't own a helmet. It's #1 on my list of snow gear to get though...