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!
Yes, made the same mistake with a best friend and it cost me the relationship. Now I pretty much just keep my mouth shut and let them figure it out for themselves. We can help them pick up the pieces afterwards!
I rather say something so they can think about it. I would hate for a friend to know such an important detail that I should know and not actually tell me. That's what good friends are for anyway, to tell you the truth when others don't. I do think there is a way to approach it and not just blurt it out though.