We knew Avalanche danger was a possibility this day and were all aware. I am reporting this for the actual rider that triggered the event. The Rider is an experienced rider and very cautious, he knew better than to climb this hill and has taken that up with other riders in the past, for some reason he wasn't thinking and calls his actions a "brain fart". There were a total of 4 of us, I had stopped in a poor position because i heard my belt starting to come apart, just as i was stepping off the sled to open the side panel another rider went by me saying Avalanche, i jumped back on my sled and started it and overserved the riders approximate position and the avalanche as it continued to break across the face from South to North. I realized I would be in the debris field so i rode away from it at that time. I stopped and observed no more activity and began searching for signs of the rider or snowmobile in which from this vantage point i could not see. As i rode through the avalanche I still could not see any signs of the rider, i began my through process on the next steps and as i rode over a small hill i was able to see the Rider's helmet and airbag deployed.