How does the fundamental tension between Finny and Gene in A Separate Peace affect their friendship?

This question is not so easy to answer. If I were to only consider Finny's feelings, I would say that the friendship is mostly unaffected throughout the novel. Then, the friendship is completely destroyed and never fixed, because Finny dies. Finny goes through the entire novel thinking the best about people. He not only thinks the best about people, he wants the best for people. He never once considers that Gene might be jealous of him. And because Finny never considers that, he never thinks that Gene might be trying to "one up" him (on test scores, for example). Finny never, ever thinks about people in that kind of way, so he assumes that everybody else is like that too. The friendship is finally ruined at the end of the novel. The mock trial points out to Finny that Gene's actions on the tree branch were not unintentional. Finny can't handle the emotional overload, runs off, trips, breaks his leg again, and dies from complications.

Gene's friendship with Finny, though, is in one continuous downward spiral throughout the novel. Gene's jealousy motivates him to bounce the branch, which, in a way, ultimately leads to Finny's death. Gene's jealousy motivates him to try and outdo Finny as well. It isn't much of a friendship if all that someone is trying to do is win. To Gene, Finny becomes less and less of a friend and more and more of an opponent to be beaten. That unspoken sense of competition Gene feels is what adds tension to the relationship, and that tension builds until it ultimately snaps and leads to Finny's death.