Everyone reacts differently to a trauma. They may not have the reaction you expect to see from someone who was assaulted.
Sexual violence is about power and control. A survivor has had their power taken away from them. It is important to help them reclaim their power by empowering them to make their own decisions, whether its making a reporting or what kind of medical attention.
Do not force someone to talk about their experience but be a safe space if and when they do talk about it. Encourage them to connect with a counselor when they are ready.