What can be done if an adult is accusing a five year old of being a molester and making threats towards him?

My girlfriend is a water polo coach and during a program designed to introduce kids as young as possible to the sport a 5 year old boy swam over a girl around the same age in order to get control of the ball. The young girl told her mother that she didnt like the way the boy touched her as he swam over her so now the mom is taking it way out of control and telling everyone she knows that the boy is a pervert and molester. Now all the other kids ask him everyday if he is really a molestor. The mom of the daughter demands that they practice on opposite sides of the water along with other demands. Recently the mother said she would have her husband and son come to the aqutorium and handle this situation if the boy is not removed which sounds like a threat to me. The mom is really starting to annoy people with her constant harrment towards coaches and the 5 year old boy. What can be done about this?