>>149227385 (OP)Cube's an asshole
Any recording engineer in this context would bite the bullet and just say something like "no dude it sounds great, keep going", or make up some non-advice like "you tried breathing from the diaphragm?" just in order to get the client more confident and keep the session going forward.
Not making any effort to throw him the slightest bone isn't just shitty on his part, it's also unprofessional on his part because it's making both of them lose time, and it will lead to an inferior product because he wasn't capable of getting better takes out of his customer.