What to do...?

Every time my fiance's 3 yr old daughter comes from spending time with her mother she comes home using profanity, which she does not use when she is with us(because we don't say bad words around her). Also she comes home mocking things that her mother says. Things like "If I'm not good, mama's going to call the police on me," or "If I'm not good mama said the boogie man is going to get me." Her mother is not excatly motherly material... but is there a way we can reinforce to her that police are here to help, and not be afraid of them, and that there is no such thing as the boogie man. I'm asking this because we ran into a police officer in the market yesterday and she started screaming at the top of her lungs because her "mama says police will get me if I'm bad." It's just a phrase her mother uses to get the little one off of her back if she's acting up. My fiance has talked to her mother about this.. but talking to her is like talking to a brick wall. Any suggestions?