By talking to Walter Cunningham as she did, what lesson does Scout show Atticus that she has learned?

Atticus had previously told Scout that people in a mob do things that they'd never do if they were alone. By calling out Mr. Cunningham, Scout makes it personal, "Hey, remember when my Dad helped you?" This type of conversation pulled Walter away from the rest of the group because he'd been singled out; she succeeded in making him an individual again.