Even though your friend may not like it very much, but you should still tell an appropriate person about her getting abused. You can approach her family and tell them what is going on, or you can talk about this with a common friend. Maybe her family members or your common friend would have an idea about how to best handle the situation.
Do not go around discussing the situation of your friend with people who are not close or interested in your friend’s life and problems. Gossiping will make you the centre of attention for a while, but you will end up losing the trust of your friend and even hurting her.
Your friend would most like appreciate your being there to listen to her while she is going through a difficult time. Try to take as much time out for her as possible. She may have a lot of things to share, express and vent.
Try to get your friend away from the abuser by inviting her over to your house. It would be a really good idea to give her a comfortable and safe environment, where she can relax and talk freely. You can also invite her to spend the weekend with you. Invite other friends as well, so that your friend can have a good time and heal from the mental wounds inflicted by abuse.
There would be times when you friend would want to be left alone and think in peace. However, do not stay away from your friend for too long, as too much thinking may push her into deeper depression.
If the abuse gets worse, take your friend to the hospital to get her treated. She may not come with you easily, but being a concerned friend, do whatever you can to get her to the hospital. Make her understand that it is for her own good.
Report the abuser to the authorities if the abuse gets too much.