There are times in everyone's life that you see friends doing things that are... silly. And as the friend of wisdom you have two options A) Tell them they are being stupid and to come to their senses. This may mean you need to take a stand that your friend won't listen to. It may lead to you having your friend go on making their silly choice without you in their life. B) You tell them you want them to be happy and you want what's best for them. Even if you know they are walking into their own disaster. That way - at least - you can be there to catch them as they walk off their cliff of choice.
You can't have both. The closest thing you can have to both is by politely pointing out your friend is an idiot. After that goes disasterously wrong you proceed to apologize for your lack of faith in their decision. Then you stand by for the fall out. The one you accurately predicted initially however, in these situations I-told-you-so isn't appropriate.
None of the choices are pretty. You have to sacrifice something in all options. So which is the best way to handle situations like these?
Whenever I'm faced with this terrible scenario I usually do B) or the combination silent I-told-you-so. I may have to swallow my pride and I may need to stand by and watch my friend get hurt. I feel like, though, if they're going to do it anyways I might as well be there when they need me. Not everyone can take advice in all cases. Some people just need to experience things for themselves. I understand that. It's still hard to watch people you love go through painful experiences that they could have easily sidestepped in their journey through life. Sometimes you can't help them avoid pain, you can just be there to hold their hand as they experience it.