Yes, it's true the guy is a bonehead. Yes, it's true that his idea is probably not going to sound even intelligible, as a final product.
On the other foolish side of it though, I recall the 60s, OK pretty dimly, but absolutely stupid experimentation was the order of the day. Many hit songs had dreadful sound, and extremely cosmic recording practices, compared with our modern clinical capturing of sound. I sometimes wonder if trying goofy shit, and finding and exploiting idiosyncrasies in gear can't still be a valid creative tool. I've at the very least had a lot of fun in doing so.
But if the man so insists, and it is not personally abhorrent to you, do the job. He who insisteth usually has the purse.
Maybe try some communication skills with this guy, and ask him why HE thinks that is the way to go. What his specific intent is by recording that way. Is it something specific that he wants his voice to sound like that you could achieve with your rack, say, that would meet his need? even just in his monitor. Who or what is trying to emulate?
Is there anything he could do to treat the room you mention... bass traps in corners or that type of thing... to mitigate the terror, as it were.
Good luck to you.