Hmmm, so he's suggesting that you can feel more details with a plastic grocery bag on your hand than you can with your bare hand? Pretty much lost my willingness to believe at that point. Also not sure why you would want to clean and shine a guitar to that extent. But then I also haven't washed my car in a long time so maybe that's just me.
Sorry, not spraying ANYTHING on my guitar that I haven't tested on something I don't value first. And I have extremely sensitive fingers from years of feeling for people's veins before I try to put needles in them. He will not have me as a customer.
I doubt the spray and the clay would hurt any high gloss guitar. He's just borrowing a very basic principle used on the automotive and jewelry industry for years and years now. On any other finish though, this could be bad for a variety of reasons.
I'd be more concerned about the wax. It wouldn't hurt any solid body guitar, but if someone were to try that on a chambered guitar or even worse, an acoustic guitar, that could cause some serious problems because carnuba wax takes quite some pressure to buff out properly and I could see someone possibly damaging their guitar just from the pressure needed to buff the wax to a proper shine.
And, if they didn't buff it properly, the guitar is going to look like crap.
===================================== One Alvarez in the hand is worth two Taylors and a Martin in the guitar store!