When to be a manager vs. a friend

I am on a team of 14 managers, and only 2 of us are female. The other female manager in my work group, who is also my friend at this point, confided in me that she is being sexually harassed by our boss, and I don't know what to do. I've tried to get her to report it, but she refuses; she thinks it will only result in backlash and having her reputation tainted as a troublemaker. I offered to report it on her behalf, but she has begged me to not get involved and for me to forget she ever told me. This is an ongoing situation, and she is trying to be tough, but I know how much it rattles her and she can't focus at work because she's on edge. She's stopped engaging in team activities or things like drinks after work and makes up an excuse why she can't join us every time because she wants to spend as little time at work as possible to prevent it from happening more. I don't know what to do here. My heart aches for her, and I want to report it for her, and I want to report it for our company so that we can have a better work environment where people can feel safe. As a manager, I feel like I have a duty to report it, except she's begged me not to tell anyone. We don't have any tools or resources like an anonymous hotline or anything I can call. Also, because this only happens in private and I've never seen it, if it gets reported, it will no doubt get traced back to her, and then she will know I betrayed her trust. As a friend I want to respect her privacy and her choice. I am feeling really lost and helpless right now, in addition to how upset I feel for her that she feels helpless, and that this is happening in the first place. How can I help her? How can I change this behavior at my company? How do I balance being a peer manager and a friend?