Many people do not tell anyone about being sexually assaulted until much later. This does not mean that it has not affected them until this point. What it does mean however, is that for many reasons they were not able to tell. It is common for the victim to feel guilty over the assault and question what they did, or believe that they could have stopped it. It is important to let them know that whatever they did at the time helped them to survive.
Sexual assault is never the fault of the victim.
You can not make the sexual assault go away, however you can assist in the healing. Believe her, and tell her that you do. Reassure her that it is good that she has told. If it has taken some time for her to disclose, try to understand why she could not tell you earlier. Stress that it was not her fault and that she has done nothing of which to be ashamed.