You need to make a decision. Is it more important to get ex girlfriend back or to save her from this horrible nightmare that she may be in. What is going to make the most difference is if she sees it as you looking out for her best interest rather than yours. You will also have to look closely at your motivation so that you don't just use this as an attempt to further your own interest. Whatever you decide, though, if you see the good that needs to be done, do it. You also need to realize that she may not want to be rescued and have to accept there is nothing that you can do.
Being the hero can go a long way in her eyes if you save her from not just an unhappy situation, but a potentially life threatening one. It is well known that once someone has been abused that it is easy for them to fall back into it. If she knows that you are a safe one that she can turn to then she will find it harder to turn away. You have to first let her know that you are a safe place.
Hopefully she will have known that you are able to take good care of her. If you didn't do that good of a job, let her know that you have changed (you better have seriously changed, though). Let her know that she can depend on you and that you will listen. That may be the hardest part but it will go a long way. It is a good thing to learn how to do anyway. Also make sure that she knows she deserves better. An abuser will thrive on making the abused feel small and that they deserve what they are getting. Her self esteem may be severely hurt. Make her know that she is special. Get ex girlfriend back from that bad place because she truly deserves better.
Your true motivation for saving her from the abusive situation will help make it easy to get ex girlfriend back. If you are trying to make it obvious that your main priority is to get ex girlfriend back then you may do something that compromises her safety. If the abuser gets an idea that someone is trying to take her away and that she might be contemplating it then the situation might get worse. Abusers are generally very jealous and have a great need to control. When they feel that their control might be lessening up they will try to correct it using whatever means they feel is necessary. What ever you do trying to get ex girlfriend back, you need to think of their safety first.
If you see that your ex girlfriend is in trouble, do something about it. It is too easy to turn away from someone in need. It can get messy in your own life but there is much good that can come from it. If you succeed, not only will you get ex girlfriend back, but you may have saved a life.
Read more in The Magic Of Making Up