Hey there,
So it sounds like you are a really concerned friend. Have you tried telling her how scared you are, and that it's not her fault that it happens? I know she might not listen, but try telling her again, be calm and don't get angry, just keep talking to her.
There is only one thing you can do, besides being there to support her. You can tell someone, if she goes to your school or lives near you. If she's in internet friend,you can try getting her to tell someone. It's not a healthy place for her to be, she shouldn't have to be afraid of her father, and it's only a matter of time before he hurts the other kids. If you think she's in danger, and she goes to your school, tell your school counselor, or even tell your parents. Let someone know that she's not safe there, but only if you really believe that she is endanger. She might be angry at first, but that's to be expected. Just do what you think is best.
If you need anything, feel free to
pm me. Take care.