How to Tell Your Client That His "Expert" is an Idiot

Esther Schindler writes: It’s a danger for any consultant, and for most inter-departmental internal project staff: To get the work done, you need to work with someone else who supplies expertise you lack. But when the “expert” turns out to be the wrong person how do you tell the client (or boss) that you just can’t work with that individual? It’s possible to do so, but it does take a deft hand. Here's one set of instructions, but surely there are plenty more you could add.