It is definatly his fault. One thing you should always do when you go riding with friends on the trails is tell each other how and where you plan on riding. Same kind of accident happened to my brother. My cousin was up in front on the trails on a raptor, my little bro was in behind him on his crf-150 and I was in last on my ttr-125. Since my bro never really rides much with other people, he was getting a little excited and speeding, trying to stay close to the raptor. When the raptor slowed down fast, my bro paniced and hit the front brakes too hard. He ended up fliping and breaking the clutch lever all the way off. He had to ride about 2 miles back to the house in 1st gear