Listening is the first and most important step. Believe what your friend is telling you. S/he may think no one will believe or care about what s/he says.
Support Getting Help
Support your friend in getting help with the situation. If asked for help, then you can find resources to suggest to your friend.
Don't Give Advice
You may want to, but really giving advice is not your place. It is her/his life. S/he is the expert. By giving your opinion and advice, there is an expectation that your friend will follow it and consequently will not be depending on her/himself. Instead of advice, offer choices such as calling a crisis hotline to talk with someone or letting your friend know s/he can talk with you again about what is happening.
No One Deserves to Be Hurt
Often individuals involved in violence (in any form) engage in self-trivialization and minimizing. Self-trivialization involves the individual putting her/himself down, putting everyone else's needs above her/his own. Minimizing takes place when your friend begins doubting her/his own pain. "It wasn't that bad." Often times minimizing includes making excuses for the offender as well. This means your friend may say the violence occurred only because alcohol was involved, stress is high, or some other reason to excuse or explain the violence.
This is a key element. It may be very difficult to see a friend stay in a situation that you don't believe is healthy or right. However, you must remember that everyone has the right to make her/his own choices. It is your friend's choice to stay or leave her/his partner, to have a rape kit performed and given to the police or not, to talk to a counselor or therapist or not, etc. What might be right for you, may not be the best choice for your friend.
Take Care of Yourself
Helping a friend through a turbulent time can be quite emotional. It is crucial that you also talk about your feelings and get support. Call someone at one of the shelter hotlines (resource hotlines) if you feel the need to talk. If you are emotionally drained, you won't be able to help your friend. So, take care of yourself too.