I actually asked a ranger that very question.
He stated to me that when ON THE TRAIL, the rules are the exact same as when you drive a car on the road, they can ticket you for the exact same things when sledding on the trail as when driving your car on the road.
If its the main trail/summer road, then it is this way in Utah also... Highway vehicle rules apply, and there is a speed limit for said trails.
My dad had a similar thing happen... First of day, heading up the trail, I and my brother get past a stretch in the road, and imediately after, my dad gets on the radio and says there has been an accident.
An underage kid had come up from the creek onto the road... My dad had no where to go and they hit.
The first thing my dad did was make sure everyone was ok... He very easily could have hit the kids leg, but didn't.
After that he and I beleive his dad, somewhat admitted fault and exchanged numbers.
After the fact his mom was upset and didn't want to pay for my dads sled... Things got ugly so he took him to court.
My dad had the right of way and the kid was at fault... There is a speed limit and the kid tried to say we were all speeding.
The judge in a way reprimanded the kid and said that if what he said was true, the damage would have been much more seveir and also that there most likely would have been life threatening injuries due to the speed at which the kid said we were going.
I beleive he was on a borrowed sled, so he had 2 sleds to pay for.
His dad was very cooperative, and his mom almost was found contemp of court.
In the end his parents had to pay... He and his dad talked to us outside of court and apologized as his mom was livid.
Had this not have been on the road, they each would have covered their own repairs.
I think that you should find out if it was the main trail.
With that and his notorized confession I think you will be fine in court.
For those that say you may loose a friend, he is no friend in my book!