Remembering can be triggered by events such as the birth of a baby, a TV program, marriage, changing jobs, starting a new relationship, death of an abuser, etc. Do not ask them why they did not fight back. People can freeze when confronted with a terrifying situation. Do not ask why they did not say anything sooner. If it happened when they where young, they may have tried to tell but were ignored or disbelieved.
Most people do try to tell someone at some time.
Do not tell them what to do. They need to be in control of their own decisions about matters that affect them. You can help them to explore options that are available to them. Do not pressure them into doing, or talking about things they are not ready to face. When they are ready, they will speak.