This sounds like one of the things I hear a lot.
Not so bright people trying to make themselves look brilliant by proving that conventional wisdom is wrong.
"hey, you know what? when you are shooting that elephant, you won't have hearing protection, so you better make sure you're used to listening to your .700 nitro express without protection! You wouldn't want to flinch, would you?
Conventional wisdom USUALLY becomes dominant because of a couple of things. It is tested by time, and many, many people, have found it to be right. Unconventional wisdom is seldom either wise, nor conventional. it's not stood the test of time. All it is is a protesting counterpoint.
We're not talking about grey areas like fast and light vs slow and heavy, this is a pretty specific point. Can a person become an effective hunter or combat shooter while wearing hearing protection at all times? Gimme a break. Of course they can. Once a trained shooter puts the finger to the trigger, the mind will be focused to the exclusion of most estraneous stimuli. A point to consider is that EVERY time he pulled the trigger on a live round, it went bang, and there was recoil. Is the degree of loudness really going to distract someone to the point that he can't function properly? Come on.
OTOH, anyone here of my age can testify to the ongoing cumulative damage from shooting unprotected. 20 years ago, I capped a giant cottonmouth with a super hot .357 round, and that one shot began a permanent tinnitis condition that just keeps getting worse.
Training with duty rounds to acclimate oneself to the recoil, noise, and other factors is necessary. Training without hearing protection so that one will be used to loud noises is ridiculous.
Did the guy perchance mention that you need to train without safety glasses, as well, because your sights will look different to your bare, unprotected eyes? If he hasn't suggest it to him, so he'll have another silly idea to share with people.