I witnessed something similar many years ago, although the fall backwards was caused by a punch. I didn’t have to do a lot, other than moniter, as, although totally unresponsive, the guy still had a pulse and was breathing. Unfortunately the guy died a few days later from the massive head injury he sustained.
For a while, like you, I wondered if I could have done more, but eventually I realised that he was probably brain dead when his head hit the floor.
At the trial the judge thanked me for my efforts and outside the court the lads mothers stopped me and thanked me for trying to do something.
As others have said, the guys family will be grateful that you acted.