Tom's rule for the day: "Never confront people with their own ignorance."
Let's say someone is in a critical position to do something. Let's further say that said person has no real grasp on what needs to be done, and isn't even approaching understanding any of the foundational pieces of knowledge that would be necessary to understand what needs to be done.
Are you justified in telling that person just how vast his incompetence is?
It won't help. What's the best possible outcome of such a disclosure? That the person would realize the insight of your comment and humbly step aside, allowing someone more qualified to act. Does that ever happen in real life?
Much more likely, the person will become an enemy. Defensive and eager to shift blame -- especially if it can be directed back to you.
The Better Approach is to acknowledge what the person can do and then work with them to help them through the problem. Teach them, if you can, what they need to know. Provide as much help as possible, subtly, without calling it such.
Especially if you're a vendor and they are an employee of your client. :o)
(Note to any clients who may be reading this: I'm not talking about you!)