I deal with victims of abuse more often than I want to, but it is the nature of what I do. How I treat this situation depends on the person seeking help. If authorities are involved, it requires an entirely different approach. I have even treated child abusers themselves (something that didn't sit well with me, but someone has to).
SDK, the first thing is to listen and make sure everyone is safe. If safety is in question, then authorities need to get involved and the victim(s) need to get to a safe environment unknown to the perpetrator of the abuse. The next thing is be prepared for many highs and lows going forward (when the victims are ready to move forward). Victims can blame themselves or even believe they actually deserve this treatment. However long the victim(s) have been in this type of relationship is how long their cognitions have been eroded. Let's say the abusive relationship has gone on for 10 years...that is 10 years worth of toxic thoughts which erode someone's ability to process information/feelings and even degrades immune system functioning (has she been sick more in the last 5 years than the rest of her life).
Be patient. You may see a little step forward and 10 steps back. That is part of the process. Anger is a central feeling in abuse and that is often directed internally.
Listen and react with empathy and acceptance. You will hear some things that will quite frankly make you sick to your stomach. I have a puke bucket in my office for these occasions. Think, process and work through emotions. Responding with violence to someone that has done a violent act to someone else (even though it is justified) will not get anyone anywhere in this life. Let the authorities handle those types of things.
SDK, this is a long journey ahead for her. I obviously don't know her and can't tell if she is rational or irrational in how she processes everything. It is important for you and others around her to be consistently there for her. Most victims I have encountered have one thing in common: they aren't looking for a person to cry with, but someone that accepts them for what they are now. You are not a counselor/psychologist so don't diagnose anything or feel the need to treat anything. Help her find help. Be the support beam that she needs and that will do more good than you can possibly imagine.
Dr. Blumperz