That owner is a moron. There is no way it handles better with more pressure. It goes against the laws of vehicular physics.
I got into an argument with a guy at work about this exact same issue a few weeks back. He tried to say you always run max psi and I told him he was an idiot. He tried to tell me me knew better than I did because he used to sell tires.
Just because one sells/installs tires doesn't mean they know anything about tires.