When a person opens up about being hurt or abused a lot of emotions can go through your mind.
The worst thing you can say is, “I’ll NEVER tell anyone”. To protect them, and yourself legally, no matter what you NEED to tell someone. This can be difficult because you feel like you’re betraying that person and breaking their trust. The best thing you can do to help them is to tell someone, or offer to go with them if they want to. If they don’t want to tell anyone else themselves, then YOU need to tell your parent, teacher, or youth pastor. Someone who is an adult in authority needs to know. Although they may be mad at you for a while, in the long run it is in their best interest that you do. They need to get out of that situation and get the proper help they need to be safe.
Remember to pray for them, ask for God’s help and guidance, and show them compassion and acceptance. It’s never that person’s fault, so don’t judge them - just love them. And reassure them that you will do whatever you can to help.