Your mate's an idiot, not to put too fine a point on it! <grin!> Where does he get the, quite frankly, ridiculous notion that oatmeal's gonna make him build more muscle (ir's no good as a pre WO carb as it's slow-release)?
As for docs, well it's simple. Read everything Mark has to say about CW and there's your answer, pretty much. Oh there's probably more to it than that (flawed research for starters) but IME docs, nutritionists and dietitians don't really have a clue (eating fat will make you diabetic's a good one - this is in relation to type 2 diabetes more than type 1, I'd have thought, but it's still nonsensical!)
They're taught all this rubbish at med school, or when they take whatever exams it is they take to become nutritionists/dietitians and it sticks (I've had it all first hand from a nutritional therapist who told me that, in order to cure my insomnia, I had to eat porridge for breakfast, snack on oatcakes and Ryvita (rye crispbreads) and make sure I had potatoes, rice or pasta for dinner! She also banged on about eating 6 small meals a day (or I'd destroy my metabolism and get fat again - y'know the old 'starvation mode' bollocks). She even told me that Dr. Atkins died because he was obese due to following his own teachings!
Needless to say, I've not seen her again since!
I can't really answer you beyond that, other than it's what they were taught (and I wish I knew where all this rubbish stemmed from originally because it's doing many people so much harm).
Sorry, I can't be more help, bud, but I figured that some answer was better than no answer at all (though there are some here who'd probably disagree with that!)