It is often said that the customer is always right. Obviously, this is suggesting that the customer should always be accommodated in whatever he or she wants. And, to some extent this is a good policy. It can often lead to a very happy and satisfied customer. But not always.
We are in a situation here where we, as experienced and knowledgeable craftspeople, know much better than the customer what will or will not work. When the customer is asking for something we know is bound to fail in one way or another, it is incumbent upon us to attempt to communicate this to the customer.
This often means telling the customer that they cannot get what they want. Or, at least informing them that what they want is impractical or in violation of some basic principal of design or construction. While it may be possible to build what the customer is asking for, it might also be doomed to failure at some future time and the customer will not be happy when it does.
I have been in situations in which the customer insisted on something even though they had been told that it was a bad idea and given real practical reasons why. I have even gone so far as to write in a clause in the agreement that absolves me of any responsibility for the failure, if and when it occurs. But even if the customer is willing to sign such a document, they will still be unhappy if the failure actually happens.
This is as close to a no-win scenario as you can get. If you give them what they want, you are set up for a fight later. And if you don’t, you risk losing the job.
Experience has taught me that the better option is to let the job go. Much less risk of bad press later.