Has someone ever called you with the world's stupidest plan? Like even if you try to be non-judgmental and "go with the flow," you can come up with 82 reasons why the plan is completely stupid. The person is so excited and convinced that it is The. Best. Plan. Ever.
So what do you do?
Well, you keep your mouth shut and let the person figure it out for him/herself the hard way. Right?! You might want to hedge around and bring up some of the more dastardly outcomes of the stupid plan, which of course won't prompt the person to say, "OMG, you are right, this could turn out very, very bad." Instead this will prompt the person to get indignant and even more attached the aforementioned stupid plan. Then you back out of the mess by saying that you were just playing devil's advocate and that you trust they will make the right decision.
Here, folks, is when you wait for the fallout. And there's a lot of fallout, so much fallout that it keeps YOU up at night. Then you think to yourself, "Why didn't I intervene? Why did I just let this happen?" Two-pronged reason that we've already discussed: 1) The person wouldn't have listened and 2) The person would have become even more entrenched.
Fallout happens and then subsides.
Person calls you AGAIN about ANOTHER stupid plan, in the same vein as the previous stupid plan.
You want to scream, "HAVEN'T YOU LEARNED?!!!!!!!!!"
But you don't. You do recount the synopsis of the previous stupid plan and the fallout. Person gets indignant and says THIS is a completely different situation than the previous situation, and this one is guaranteed to end in blissful happiness.
This is the point that you simply state, "Let me know how it works out for you."