The litmus test for me is always "Is there danger to that person or someone else?" If the answer is yes, you should contact a mental health professional or law enforcement, or direct the person to a emergency room at any hospital. Safety is the first concern. I have been a school counselor for more than 30 years, I have had this situations dozens of times. I have always acted on the situation and never regretted even one time. If in doubt, go to a counselor or clergy and ask them to help.
Sometimes a person is cheating or stealing or something that has a lower level of risk, in those situations, you must let your conscience be your guide. There may be more than one correct answer.