I think it depends on the subject. I made the mistake of telling my now ex best friend that her husband was trying to get in my pants when I slept on their couch one night. It was something she needed to know, but it back fired on me. For years, she believed him when he said it never happened. Then the same thing happened when he tried it with her cousin. Once she realized I hadn't lied, she called me right away. We are close again, thankfully. You should really choose how you tell the person, and be prepared for them to ignore your advise, or expect it to maybe not go over so well. You have their best interests at heart, so explain that to them before you tell them. Good luck!

This. If I care about them and am close to them, I'll tell them.