Sorry about your friend, how is he recovering?
I've had a few friends finish a fun day early with an ambulance trip. One was right in front of me (I rode between him and his bike while I was trying to stop) and another was lost down an embankment for about an hour before we found him too injured to climb out.
Anytime we ride, we run the risk of not finishing safely. More people means more chances of that happening. Less experienced riders want to ride with somebody faster so they can learn and improve, but sometimes they push too hard or freeze up. It's a difficult choice to decide to be the rabbit - you could be helping a friend grow in the sport or you could be leading him to a trip to the ER. Ultimately, every rider's responsibility to get home that night is his own, not yours. Riding solo and riding with a friend or group both have their pros & cons, but you can't feel responsible for what other riders do. At least, you were there to help him and make sure he got transported to a hospital - had he been solo...
I'm retired now, so let me know if you need a helper to go recover his bike.