In the book A Separate Peace Finny does not see Gene's jealousy over him. Instead he identifies that Gene has the same strong feelings of friendship for him as he has for Gene. He has bonded with Gene and looks to him as a trusted friend.
When Finny falls he probably knew subconsciously and even spoke the words, but he could not mentally fathom that someone who he cared about so much could do such a thing. Therefore, he dismisses it. Overall Finny is the essence of everything good in the book. We know he is mischievous but he is also genuine.
Books do not have to be about realistic reactions. However, a trauma like Finny had experienced could skew one's thought process as well.