What you do now is apologize for your behavior but not for your message. Let her know what you told me — that you care about her and that you understand her better than she may realize because you see a lot of yourself in her. Express your regret that you lost it. Then gently and lovingly ask her if she understands why she treats the father who loves her the way she does. Ask her if she needs help finding help. Be prepared with a list of therapists if she takes you up on your offer.
If she rejects your overture, you’ve done all you can do. Then it is up to her dad to confront her if he is upset by her behavior. It continues at least partly because he allows it. There may be good reasons why he does. You don’t know unless he decides to share it with you.
If he does want help with the situation, encourage him to see a family therapist for support and practical help. You can’t be the “therapist” and shouldn’t be. These two need you to be a loving and supportive friend. Let the therapist do the therapy.
I wish you well.