Just be there for her. The most important thing that I could have done for my best friend who was being abused was realize that I could not take control of her or the situation. My first (and second and third) inclination when she finally told me what was going on was to hop on a plane, rent a truck, and drive her and her stuff back across the Mississippi as far away from him as possible. But I realized if I did that, if I was her "hero", if I made those decisions for her, not only would they not be hers and most likely not stick, but then how much better am I than the person abusing her--taking away her freedoms from her, making her feel like she has no control or say in her life, not valuing her decisions, etc. I got very good at meditating. You also need to strike a balance between listening and being supportive, but not indulging. Draw your own lines too because you are in for a long ride and you will get worn out and impatient, but stick it out. It is worth it. You can also get her information (but be careful to convey this information in a safe way, perhaps sending it to her work rather than home address).
And remember, a woman tries to leave on average seven times before she finally walks out for good. (I called my DV shelter and talked to someone too, that helped me)