In chapter eleven, Brinker and a few of his buds come into Gene's room and pretty much kidnap Gene and Finny, taking them to the mock trial. Gene and Finny don't have much choice in the matter; Brinker's friends
"half lifted us half-roughly, and we were hustled down the stairs."
So, they are roughed into going to the trial.
At the trial itself, Leper is brought in as a special witness, and in his very descriptive witness of the event, basically confirms that Gene did in fact cause Finny to fall. He describes the two figures on the tree moving like an "engine" with two pistons, one sinking first, then the other one sinking. This makes it pretty clear that Gene jounced the limb, causing Finny to fall. This, for Finny, shouldn't have been too shocking, since Gene has tried to tell him this in the past. However, Finny has been living in denial. Leper's confirmation forces him to accept what had happened, that his best friend in the world had purposely ruined his leg. The news is so shocking that the typically serene and friendly Finny curses at Brinker, and flees, crying. He has finally had to accept that betrayal and cruelty exist in the world, and worst of all, from his best friend.
I hope that those thoughts helped; good luck!