Gene tries telling Finny what had happened pretty soon after the accident; in the infirmary, he tries to gather courage to do it once, but ends up chickening out because "Dr. Stanpole came in before [he] was able to." Then after the summer break, at the end of chapter five, he goes to visit Finny at his house, and is actually able to confess. Finny's reaction is actually quite interesting. He instantly denies it saying,
"Of course you didn't do it. You damn fool. Sit down, you damn fool...I'm going to hit you if you don't sit down...I'll kill you if you don't shut up."
His first reaction is pure denial, and then he gets almost violent, and very angry. Finny, the least violent and angry person in the book, threatens to kill Gene if he doesn't stop talking about it. This is one of the few moments in the book where you are going to see Finny upset at all. His reaction is understandable; his best friend just got finished telling him that he had caused intentional harm that had pretty much ruined his life. Finny didn't want to believe it. It would mean too much. It would mean that the person he cared about most had purposely destroyed his life. That's a pretty depressing reality. Gene realizes this, and says,
"It struck me that I was injuring him again...this could be an even deeper injury that what I had done before."
Betrayal by a close friend is a pretty serious wound. Gene decides to take it back and then quickly changes the subject.