I need some help with a Parental issue. May someone give me some advice?

Hello Everyone. I have a very important question that needs some serious thought and help with. I know someone whose mom has a boyfriend who she lives with. Her child, (the person I know) is constantly telling me about how the boyfriend treats her differently when her mom isn't around, as opposed to when she is around. Such things as yelling more at her, or hazing I guess you can say. Blocking her from getting her chores done, things of that nature. She says she tries to address it to her mother but her mom doesn't want to have it. She says her mom says; "He is only joking sweet heart, lighten up." This guy really does treat her like she is low. How can she get her mom to believe her?