How can you prove verbal slander?

My daughter called child services because she believed that her best friend was a victim of abuse. Now the mother of her friend has started spreading rumors to discredit my daughter. It has gotten to the point that my daughter is ashamed to go anywere near her friends residence because she then has to listen to the rumors being repeated as fact from neighbours of her friend. What does the law say about this if anything. (we live in Nevada)