Domestic Abuse Issue, I need help!

One of my best friends is dealing with physical, mental and emotional abuse. I'm posting anonymously, because I've told her about this app, and I don't want her to know it is me. I have read domestic abuse websites and what to do as a friend for her, but I still feel so helpless. She has changed so much since they have been together and I'm seriously worried for her. He has infiltrated her brain, he has complete control over her and everything that she does. I'm one of the few friends that she is "allowed" to see. Their fights have become public, I've stepped in, cautiously and with reason, on a few of them because he was obviously intoxicated and angry. His anger is always pointed to her and he tries to manipulate me and the situation while everything is happening. I'm severely worried for her safety and I am worried that he is going to severely harm her. She has left more the several times and has returned to him (which I understand is normal and takes an average of seven times before someone will leave for good). Are there any practical steps I can take, or firsthand knowledge that would be beneficial? I am just supporting her emotionally right now, just being there for her when she needs me. It is so hard. My heart is in shambles over this. Has anyone who has been in this position or been a survivor of domestic abuse have any advice for me? Even objective opinions will help. Thank you.