You shouldn't tell on others.
Jack sees his coworker take supplies from the workplace which is not allowed.
Jack wants his coworker to return the supplies to the workplace.
Jack talks to the coworker privately and tells them that they should return the items.
Jack tells the supervisor that the coworker has taken things from the workplace.
Jack's coworker returns the items but does not get in trouble for taking them.
Jack's coworker is forced to return the items and is then fired.