The moment of grace is after Mrs. Turpin has complained to the black women about the girl at the doctor's office, then she goes down to the pigpen and shouts at the hogs.
The problem is that she interprets the college girl in the doctor's waiting room as insulting her, when she was the one insulting everyone around her by the way she was talking about blacks in a very racist way. Up until her moment of grace, she could still frame her world according to her racist ideology. However after that days events and she is alone at the pigpen, she looses it. Her world view falls apart. She screams at the hogs. Then no one answers. It is only the silence of the night. It is as if finally she is alone with her obscene and racist world view and she has to look at it.