You are walking around a mall, and you see one of your good friends walk into a store and steal something. Would you not tell anyone, and continue to be friends with him knowing that he stole something? Two days later when you are in a different mall, you see this friend again steal another item from a store. You now know that your friend is a thief and will continue to steal. Do you go to a higher authority and inform them, or do you forget about it, and continue your friendship?
You go to your friend, you tell him that you saw what he did, and you offer him your assistance. How can he get out of stealing, who should he consult with, how can he return the things he stole, and so on.
If that doesn’t work, you go to a higher authority.