Wasn't gonna comment on this thread anymore cuz it sounded fairly "wrapped". But what the heck...
This is definitely a trust issue. He obviously sees the DM as an adversary, not a referee, and he is assuming the DM is also seeing him as an adversary.
I concur...for my 2 coppers worth of armchair psychology. Based on what we've been told, this certainly sounds to be the case.
...I think the DM should be involved. Somebody, probably the DM, should explain again what the role of the DM is. They might also mention that the DM can't "win".
Concur, two time.
If the DM felt like winning he could simply throw something at the party that's ten levels beyond them, or he could simply say, "you all die."
I must admit, many moons ago, I myself was guilty of something LIKE this. The group was completely off topic, not off "rails"...I wasn't railroading them, like "off game." There was all kinds of in-party drama occurring and no one was really playing "in character", simply arguing with each other. It was a good sized group (7 players, 8 or 10 characters, I think). Not low level or inexperienced players, either. We'd been "playing" for over an hour and nothing had happened.
I simply plopped down a dozen ice devils in their midst and started combat.
They were, collectively, incensed. "What?! You can't do that!"
I said, "I can. And I did. Roll initiative." A couple of bloody rounds later I said, "Now, can we move on with the game?" The ice devils teleported away and that was that (I, of course, returned their status to where they were before the devils arrived/removed their injuries.) Everyone was paying attention from there on out and we got to play.

And had a great time for the remainder of the night. Cheap trick, but...sometimes players need a..."focusing" hand..a glimpse of just *how* wrong things can go.
Not saying that your DM needs to take a similar tack. In fact, that tack with this player would no doubt prove catastrophic. Just illustrating that them's the reality of the game.
This player needs to understand that the DM's job is to make a world that's fun and challenging for the players. It is not his job to win. A good DM "wins" when everyone is having a good time.
And, I concur three times. That's the golden fleece right there..."A good DM wins when everyone is having a good time." and, I daresay...it reverses also, the players contribute to the DM's good time as well by challenging his/her story, expectations, imagination (i.e. out thinking the DM's preparedness), etc. A game is as good as ALL of its participants (DM and players.)...so if one of the participants is not contributing to the overall fun and enjoyment...then, there's only so much that can be done before the boot is appropriate.
Again, you can be good friends and NOT be good RPG companions. And as "mature adults" everyone involved should be able to recognize that.
--SD