Now, I was 22 when this book came out. So I had SOME level of adult sophistication, and I recall being wise enough to take anything that was in D&D with a HUGE grain of salt. So I never thought OA was in any sense a comprehensive, or accurate, reference to anything. In fact I recall comparing it to how things were depicted in the game 'Bushido', which a particularly oddball guy in our group ran religiously (he'd lived for a good while in Japan, but he wasn't Japanese, only THOUGHT he was). I didn't think that was super authentic either perhaps, but it was (at least as he presented it) probably much more so than OA.
Well, Bushido (we are talking about the FGU one, right) was arguably a bit better researched than OA. Also, by being very explicitly set in a Fantasy Japan, with no pretensions of pan-Fantasy-Asianism, it could easily steer clear of the cultural pastiche trapfalls. However, there were still some trapfalls of historical pastiche - for example, Sôhei and Yakuza basically do not belong in the same game.
That said, there was a number of idiosyncrasies and plain errors in Bushido as well. Examples are the divide between "Samurai" as a class and "Bushi" as the profession of fighter, when in reality "Bushi" basically means the same as "Samurai" and is indeed the more common term in historical Japanese; the mischaracterization of "Shugenja" as Taoist elemental wizards, the "Gakusho" and various other misnomers and misspellings. But again, this game is even older than OA.
However, there is one important aspect here: You speak of your enthusiastic "not-Japanese" friend's appreciation for the game; nowadays many would probably just call him a weeaboo. (Or has that term shifted to positive self-identification like Otaku? I don't keep up with these things.)
The thing is, he was in any event likely more qualified to judge these things than the two podcasters who kicked off this whole nonsense. He was also likely more qualified than some random US citzen who happens to have Japanese grandparents.
It is annoying and baffling to me how people who are up in arms about this issue somehow think that cultural and historical expertise are genetic traits. It is doubly annoying when they don't understand how this is different from representation in movies and TV. The fact that a monolingually English-speaking American whose last name is, say, Huang, is not inherently any more qualified to write a supplement based on Three Kingdoms era China than a monolingually English-speaking American whose last name is Smith should be obvious to anyone. The fact that an American of any last name who is literate in Chinese and has a degree in East Asian History is more qualified than both of them should also be obvious.
Of course, it should be likewise obvious that Emma Stone is not qualified to visually represent a character with Chinese and Hawaiian grandparents. (Even for people who understand you cannot get too nitpicky with who you select to represent such varied ancestries.)
That is what truly annoys me about this whole "scandal".