Remember that it is essential for a rape survivor to regain control of her life and that she must feel that she is able to make her own decisions. It is important for you to allow her the time and space to make decisions, and then support them even if you don't agree with the decisions she has made. This might mean waiting before you proceed with an investigation, or letting her have some time to think it over. Remember that she may have to testify in court about what happened to her, and that's not a simple or easy thing for a rape survivor to do in our society. If she feels guilty for not fighting back, tell her that no matter what she did when she was raped she acted in the best manner she could.
Fear often paralyzes people.
Tell her you are glad she survived, and submitting to an assault is a form of self-defense too. Tell her that you do not believe it was her fault. No one deserves to be raped. This is especially important in cases of acquaintance rape. Tell the survivor what you would have done.