You just have to tell her. Be firm. I had to do the same thing with one of my son's friends. I knew this boy was trouble from age 8. Just something about him. Once he and my son turned 12 this boy really WAS bad news. Disrespectful. Starting fights. Constantly yelling when say he was losing at a video game and sometimes even cursing. I told my son and the boy together, that the boy was not allowed at our house again. He was never to come over and he was never to call the house. My son didn't like it but I didn't back down as this boy's behavior was horrible and pointed it out to my son, who also new in the back of his mind this boy's behavior was not acceptable.