What general statement is Knowles making about the "nature of friendship" and "true virtue"?
In my opinion, Knowles is trying to use this book to say that people need to be less selfish and insecure. He is saying that these negative attributes cause conflict on the individual level and worldwide.
So, for him, true friendship would consist of appreciating another person's qualities without feeling jealous of those qualities. This is where Gene falls short -- he is too jealous of Finny.
Similarly, true virtue would consist of accepting one's own good and bad points. Virtuous people would, perhaps, try to improve on their bad points, but would not envy those who seem "better" than they are.