Hate to stereotype GAF but it's no secret that GAD is mostly guys. I'm a guy, too, but as a guy whose closest friends have been girls since high school, I will say this:
Guys tend to think in terms of action-logic wheres girls often think in terms of emotion-logic. People in this thread seem to want the husband to know because Action X needs to be done. People in this thread want the cops called so Action Y needs to be done.
But the woman in question might not be telling her husband or calling the cops because she needs to get her emotional bearings before deciding how to deal with it. As guys we think in terms of urgency and safety for those who need protection. But often, women are thinking in terms of emotional readiness and having the inner strength to deal with it. Strength to women often means ability to endure the circumstances. Strength to men often means ability to change the circumstances to make them durable.
Not saying that the husband shouldn't be told or that the cops shouldn't be called, but I do think it's not exactly respectful to the victim to simply write her off as incapable of rational decision-making and therefore in need of being rescued by her male friend who can think more rationally. What is "rational" to us guys is often stupid in the eyes of women who are thinking in terms of the proper way to deal with something that will have longterm emotional affects. We men often think in terms of immediate tangible results to improve the situation, defined by who's in jail, who got away with what, and who needs protection. But women aren't necessarily looking at the situation in terms of who got away with what crime and who needs protection. In dact there are times they need their friends to specifically not do anything, because they need to know that THEIR actions and thinking solved the problem, and that they didn't need rescuing, and thus giving them long-term confidence to deal with anything similar if it happens again.
Women often come to their friends, male and female alike, for the understanding and support it takes to help gain the strength to endure--not necessarily because they want to be rescued or have anyone try to solve their problems for them. Once on this more emotionally stable ground, they will see if they need to make changes to the situation. But they will do that on their own terms and when they feel they know what will lead to the best long term emotional health.
In short, I'm saying to the OP that my opinion is: Be there for her and do nothin more than what she asks. Not because you might lose uour friendship with her if you do anything, but because what you already do for her is what she values in your friendship now.