The most important thing you can do is listen, believe the victim and affirm that it is not the victim's fault. Some days will be harder than others, but it is possible to heal from this type of trauma. Check in with yourself and be aware of your feelings. Often times it can be stressful when someone close to us has been sexually assaulted. Know that there are services available for you too, as a support person dealing with an assault that has happened to someone close to you.
CASA's services, including the Crisis Line, one-to-one counseling and support group, are for survivors and those who support them.