I saw this a couple of times when I was in high school. It's heartbreaking to know your friend is in a relationship like this and I can see why you want to try and stop it.

The one thing I learnt was you cannot force them to get out of this mentality if they're not willing. I've celebrated my friends finally dumping their creepy bfs just to run into the arms of another. One thing people hate is to be told that they're making a bad decision because they probably think you don't respect their opinions and choices.

Do not tell her parents. You will most likely ruin your relationship or at least taint it.

Keep your opinion firm but do keep reminding her that you will always be there for her. I wish I could find any more proper advice for you but she needs to figure out that what she is doing is wrong herself so she won't get into something like it again.