Well I had a talk with him. In his opinion, flawed characters are simply more interesting, and more organic. Not every Fighter in the world is going to have 16 Strength, and there can be cool stories about someone not well suited to being a member of their class.
I agree with this premise, but pointed out that the game system doesn't take this into account, and really never has- sometimes there are ways to mitigate strange decisions, like using Dexterity instead of Strength, but a character with 18 (38) Strength is always going to outperform one with 13 Strength even in AD&D.
He came back with "stats don't matter in AD&D", and I reiterated to him that yes, they really do. A Thief with an 18 Dexterity functions like they are 2 and a half levels higher than one with 15 Dexterity, and they level up faster as well.
Just because a concept is playable, doesn't mean that it couldn't be more viable. His response is that a good DM should be able to make the game balanced and fun no matter what characters are made. My reply is, maybe, but what recourse does the DM have? Either they fabricate scenarios just so the misfit can shine, weaken the encounters so the stronger characters shine more, come up with some way to nerf someone just for following the build advice in the PHB, or give shiny rewards to someone for playing a less capable character, like more xp (he really doesn't like the idea of everyone leveling at the same rate).
"So let me see if I have this right. You're saying that because you allocated your ability scores strangely, you should get advantages to end up being just as good as everyone else is. So if player A has 18 Strength and 10 Wisdom, and you have 10 Strength and 18 Wisdom, you should get, say, Gauntlets of Ogre Power, so now you have 19 Strength and 18 Wisdom and end up more powerful because of it?"
"Sure, like how Wizards have to suck for 5 levels and then get strong."
"One, that's no longer how any class works, and two, it's one thing in AD&D when everyone carries the Wizard because they know he can eventually do things no one else can. You're talking about a group carrying a bad Fighter so that they can eventually get a good Fighter, when they could have had a good Fighter to begin with!"
Then we got into how his roleplaying informs his tactics. "Nobody minds someone who roleplays. But here's the thing: you will ignore advantages on your sheet, and act in a way counterproductive to other people's fun. This isn't the You show, starring You. Everyone else contributes equally to the adventure, and if you aren't doing your fair share, all that excellent roleplay hides the fact that you're an anchor."
And finally, I put him to task for his disparaging remarks about the other players and the DM. "Just because if, you were the DM, you think the game would run smoother, is no reason to get bent out of shape. Unlike you, this guy hasn't been running games since 1980; it's just unrealistic to expect such a high standard. And you were the one complaining that you wanted to play D&D!"
"Well, this isn't D&D. It kind of looks like it, but it's not the same."
"naughty word. Was the red box not D&D? Was the Immortals boxed set not D&D? Was Players Option not D&D? Was Spelljammer not D&D? How about those Ravenloft supplements that have guns, or all those historical green books you love so much? If all of those are D&D, then you can't say this isn't D&D because some rules or assumptions about the game have changed. You don't have to like it; I mean, there's lots of stuff about all the previous versions of the game I miss, and a lot of stuff I don't like about this version. But it's what people want to play; we tried to convert the guys into playing AD&D, and we failed to show them how it's an improvement. That's on us.
But there's always the possibility that 2e wasn't as good as we thought it was, and it's just nostalgia that makes us think that way. If you want to quit the game, that's fine, I wouldn't expect someone who likes to play Cyberpunk to automatically love Shadowrun. But I think you owe the other players an apology before you do. And I never want to hear you complain about not being able to play D&D again."
Maybe I went a little too far, I can't be sure, but the words were said, so I can't go back now. He said he'd apologize and try to keep playing; we'll see if anything sticks.