Should we turn him into the cops?

my friend J and i were spending the night at our friend Lisa's house last weekend. the three of us were staying up late listening to music. when we made too much noise lisa's dad came in and asked to talk to her in the hallway. we already knew she had a bad relationship with her parents and that this wasn't good. we heard some yelling and then a huge banging sound. when lisa came back into her room she was crying and J and i noticed that her shoulders were bright red as well as her collarbone. after a few minutes she said that her dad had pushed her into the wall and even threatened her with a knife once. my friend and i are so scared for Lisa, the last thing we want is for her to get hurt, but we're worried that if we tell the cops and they take her dad away it may be even worse for her, especially since she has a younger brother. we just want to see her happy, but we have no idea how to handle this.