Advice please

I'm in a realtionship and the man I'm with his daughter comes over on weekends. I have tried my best to make a connection with her but she is so bad. She cusses as in tells anyone she will beat their ass and doesn't listen to her dad. She is only 3 like what in the world. Her dad says it's his baby mama fault she acts this way but he won't discipline her. Idk what to do but I dread his kid coming over because if I have company she will cuss and be so disrespectful and I can't do a thing about it because I'm not her mother. What do I do?