The first priority is safety when helping someone who is a survivor of sexual violence - help them find a safe place.
Be attentive, calm, and non-judgmental; allow them to share at their own pace; you do not need all of the details.
Disclosing this information can be scary; sexual assault can happen to anyone. Let them know that it was not their fault and that you believe them.
Survivors have the right to make their own choices. Help them identify their needs and concerns; discuss their options (medical, counselling, reporting, etc.) and support the choices that they make.
Having someone disclose to you about a sexual assault can be deeply impactful and takes time and energy. Seek the support you need, such as a counsellor. It can sometimes be helpful to call a support line to debrief.