What can we do? Very upset!?

Yesterday at church ( my family and I belong to a small R.C. church) my perfect princess became upset. Sitting two rows up was a girl in her school, who has Prader Willie syndrome ( and goes to my angels class for English time) sitting with her mother. My husband and I got her to calm down for the mass. After the service, we went for coffee hour. The young lady was there, and parked herself in front of all the food, stuffing her face as fast as possible. If someone tried to get something, she would scream. My perfect princess was do upset! She went to the girls mother and demanded that she take her fat cow home. Her mother looked shocked at my daughter and yelled at her! My angel! How do we get this mother from coming back with her daughter? No one was able to enjoy the coffee hour, and my angel was embarrassed and humiliated by seeing this girl. My perfect princess stated that today at school she will give this girl a punishment.