The first thing that should be done is to get the woman out of harms way. If this means calling the police, it should be done. If the family attends a religious body, then they, too, should be notified. Often times, we need to realize that we do not have the resources alone to stop things like domestic violence.
Another option is to have the couple seek marriage counseling or some sort of other counseling or therapy. Again intervention from a professional might be the only way to break the cycle of violence.
On a more practical level, you can always be there for your friend. Friendships go a very long way when it comes to hard times. Even if you are there only to listen, this can make a huge difference.
Finally, there is some sage advice online concerning spousal abuse. I will link two very good sources.
First of all, make sure the woman is in a safe environment. She is in a vulnerable position, and it may be a bold move for her to admit to the abuse. Check for churches in your area that offer help/shelter to abused women. I know that ours offers not only a safe place to stay, but counseling and step-by-step help to get the woman to safety and her life back in order.
Womenslaw.org gives a state-by-state directory of domestic violence shelters throughout the US.
The National Domestic Violence Helpline in the US is 1-800-799-7233 (SAFE)