There’s a reason she did not go to the police. It could be fear, lack of evidence or any number of other things. Before you make a decision, you need more information. This can be tricky because it is obviously not an easy topic to discuss. That is likely why she has not told you or many others about what happened. She may not be ready, willing or able.
By going to the police, you could make things worse and you don’t want to take that risk. Imagine a scenario in which you went to the police and the alleged perpetrator became angry and attacked her in retaliation. This is not outside the realm of possibility. You would never want to be in the position of having caused that sort of outcome.
I don’t know the nature of your relationship with her. Try to gauge whether or not speaking to her would be the right thing to do. Remember, she chose not to tell you. She chose to tell some but not others. She has not asked for your help or for your advice.
I understand your desire to help but she has not asked for your help. You might consider contacting a therapist to help you make this delicate and important decision. Good luck and please take care.
Dr. Kristina Randle