Acknowledge and support her feelings. She is entitled to them. Help her to feel safe. Accept that the sexual assault is not her fault . If you feel that it was, do not tell her this as it can be destructive to the eventual healing process, and what is more important, it is not the truth.
Sexual assault is humiliating and embarrassing.
Allow her to set her own pace in revealing what happened. She may never want you to know everything, so do not push her more than she is willing. Do not assume that she is hiding something from you. Do not criticize her for how she survived the assault. She did the best she could in the worst possible situation.