Joe's advice was sound . . talk to anyone with expertise and understanding who isn't going to intervene without the woman's agreement.
Beyond which . . Don't call in "the authorities", in any shape or form . . all you'll do, most likely, is alienate the victim from you, leaving her still open to the abuse but without the support of your friendship.
Try to see a bit more of her than you have in the past, and to make sure her other reliable friends do the same - so if she reaches a point of wanting/needing to talk there's an improved chance of her having the opportunity to do so.
Never lose sight of the fact that, ultimately, it's her life and so her choice; however difficult it is for you, denying her that choice is no help to her. She's going to detect folks' concern for her, but try to tread gently so as not to pressure her.
If you judge you can get away with it and if a suitable opportunity presents itself then you might gently offer practical "help opportunities" - "If you feel it would help you can ring me any time" (clearly only if that's realistic!), or even contact details for local "help" organisations, if you feel she might not know them. But go very gently . . .
Prair, if you have such beliefs. And, above all, patience and sensitivity.
Good luck!
G.
G.