What do you do?

What do you do when you think someone close to you is making a huge mistake? What do you do if you think that they're building a trust in someone who you don't think is trustworthy? What do you do if you think that your friend is too naive to see what's really going on? What do you do if you think that they're going to get hurt? Do you let them do it? Do you let them make their own mistakes and learn from it themselves? Or do you try to stop them? What if you're wrong? What if your friend is right? What if they know exactly what they're doing? What if you only end up hurting them by making them feel like you don't trust their judgement? Tell me, what do you do?