The only thing that you can really do is be there for her, no matter what. She really needs a good friend, not just now, but always. However if it starts affecting you in some way then the best thing is to just not be as close anymore.
That said - I know from experience. My first cousin and I used to be the best of best friends. Then she started dating this guy, he was emotionally abusive and physcially, on a couple of occasions. If she asked my opinion, I gave it. I was there to listen to her, b/c I didn't want to lose her close friendship. They started to live together.
Awhile later, he was throwing things around when he would get angry - she called me crying and asked my opinion. I told her, if it were me, I'd get out now. She kept saying I don't know, I don't know, what do you think. I guess I pushed my opinion on her, and she finally said ok, come get me. So my husband and I borrowed a truck at midnight, and went get her and her stuff. I gave her my daughter's room for her and her cats.
The next morning, she went back to him, and our friendship has NEVER been the same. I don't think she wants to admit to me that that is the reason. I think that he, although a better person now (I think anyway, I'll get to that in a bit), doesn't really want her hanging around me anymore b/c of it. I don't think he outright has told her not to hang with me but has expressed an opinion here or there, and b/c she is faithful to him, goes with that.
They are now married, and he has really cleaned up his act as far as I can tell. He still has a temper but controls it better, and things are great for them now.
I really wish I could go back in time and not have pushed her to get out so much ... maybe things would be different and we'd be close again.
Now, my sob story told - if it starts affecting you emotionally or whatever, I would flat out say that you can't remain close to her if she continues on with this guy. You know she is lying and can't deal with it, and when she is better you'll be glad to be back in her life.
On the flip side, if you can handle it, just be there for her. Give her your opinions gently, plant seeds in her head. Do things with her that will help her see her worth.