Well, I'm kind of stuck. I know this novel pretty well, and I spent a lot of time looking for this phrase, and I can't find it anywhere. However, Atticus does use the term "polite fiction." If you're sure it is "polite friction," please ask again, and we'll see if someone else can help you with it. However, if it is possible you meant "polite fiction," Atticus uses the phrase in Chapter 15, when discussing the Ku Klux Klan. The term means a story people know is false, but which they allow to be taken as true for the sake of politeness. In this case, he's referring to the idea that protecting the (sexual) virtue of (white) Southern women meant they needed to take the word of a white woman when it would cause a Black man's death.