Answer:
i personally wouldn't do anything that may cause any sort of harm. it's best if we plan carefully and think about any possible outcome.
Depending on the situation and the type of person I would probably gather as much info against that person as i could. I'd use hidden recording devices if possible or use my phone to get proof. if the problem also involves law than I would secretly get a lawyer (if he or she is a friend that would be the best but be careful cause you can't trust anyone and make sure that person won't betray you).
don't forget you can never fully trust anyone so always be cautious about others around you.
also don't forget this is what I think i would do. it is not a definitive way to help