I agree that if that trail is red then it is the toughest red I have seen. If you were first on scene then some others with you would have helped I am sure. No excuse for riding past a fallen MTB’r and not checking if he is okay. No excuse for that in any book as far as I am concerned. It sounds like a bad off.
When you were on scene it would have been good had at least one of the riders asked if all was okay. Again this is no more that a common courtesy to a fellow rider. You may have waved them on or you may have asked for help. Even though his mate had “gone for help” I would still have called the emergency service based on your description of the injury. His mate would have taken at least 10-15 mins to get down and that is if he made it down okay with all the adrenalin flowing.
Healing vibes to the injured rider. Sounds like it will be sore for a while.
How was his bike, before anyone else asks.
Well done BTW for doing the right thing and helping.