Convince someone that an app cannot be done [closed]

Question

I am profoundly disturbed by a request that asks me to develop an iPad app to measure the actual size of an object using camera.

It is simply not practical.

Translating 2D object into 3D is never easy. I either need extra hardware, or have to make a lot of assumptions (which will almost never hold true). Regardless I will need time.

However, the person who requested me to develop the app is adamant of his vision. And he tries to show that he is right by listing a number of existing apps that do similar jobs. He does not realize that all the apps he refers to has a rating lower than three stars, which means they probably do not do their job.

How can I convince this person that such an app simply cannot be done in a practical, single developer fashion? Is there a PhD thesis I can use as my defense?