You must be referencing chapter 5 in A Separate Peace. Gene goes to see Finny after noticing Finny's absence for days. Gene had been encouraged by Dr. Stanpole to go see Finny. Once there, he finds Finny looking awful, but ready to talk.
Gene tries to tell Finny what happened, but backs down and instead asks Finny what happened at the tree. For a few minutes Finny makes it seem as if he thought maybe Gene bounced the limb on purpose, but then confesses he feels bad for thinking that way. Gene can't quite bring himself to tell Finny the truth even though he wants to. Dr. Stanpole comes back in as Gene is just about ready to give Finny the truth, and Gene feels off the hook. By failing to say anything, Gene consented to agreeing that nothing foolish or of ill-will happened on that day.
From that point on, Finny put it out of mind that Gene may have done anything.