One thing I didn't understand at all

I'm not sure, though Frank did say something like "they promised to be my friends, if I kept a secret". I don't know what the secret is but it seemed like the fat guy still wanted to be friends.

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The Secret that the white couple who visit wish their mixed-race friends to keep is . . . that the two couples are friends. The whites don't have the courage to be non-prejudiced publicly in 1964. Obviously, here at the beginning of 2017, much of the U. S. still feels the same.