This is a long and hard one, but let me make an attempt at unraveling this.
Players (especially ones new to the hobby) don’t know what they want
Definitions, the game
It has been my experience that many people are all “wooo, roleplaying, yeah I’m into that!” but when it comes down to it, they mean very different things. F.e. no matter how much I used examples and descriptions and whatnot, I still recruited people that were just not into my style of GMing / roleplay.
For some, roleplaying is acting in character and living in a semi-realistic world with harsh consequences even if those consequences could not be seen ahead of time (see: Game of Thrones first 2ish seasons).
For some roleplaying is like the above, but consequences are always warned about and known as meta knowledge (metagaming, as opposed to popular sentiment, is not necessarily a bad thing if all people involved are ok with it and use it appropriately). (see: Marvel Cincematic Universe – deaths are rare and important, action is cinematic rather than procedural)
For some roleplaying is a game to win, a figure in the world that they use like a chessboard. (see: PC Games, usually)
For some roleplaying is having super dramatic and loser characters at the beginning who hate the group and go through a proper arc. (see: Origin Story of Stark, f.e.)
And then there is allthethings in between.
How does your problem player feel?
You’re right, your problem player kind of wants a different game. He may or may not be used to games like Baldur’s Gate, Planescape Torment and whatnot where you savescum to your hearts delight to try out all the different possiblities the game provides you with. You messed up? No worries, use an earlier save.
It fits with the playing keep-away bit, the getting upset when things are not perfect and yes, even with the pouting.
Imagine you are him. You are excited to roleplay together but also kind of busy and you build a character with the rules as you understand them. You feel like you did great and are once more excited and then you guys come in and tell him he did everything wrong (not that he necessarily did, but we’re talking about feelings here). Fine, you suck it up after a while after all your hard work (again, his perspective) was for nothing and then you go on and get excited about beating the odds together with your group! Yay!
But what is that? Your plans suck? You are always made to feel shitty by the GM and that other player? In front of your girlfriend?!? The fuck! He thought you were supposed to be a team and now he is being made to look like a fool.
Of course he will always be defensive with the above feelings, right?
Especially if this guy is dominant in the friend group in general, he naturally assumed he was going to take the lead.
His way of playing is, first and foremost, valid! People play like he does and have loads of fun. (Especially the blank-ish canvas is definitely valid, especially especially if you start out as relative nobodies and want to have a grand hero arc.) He wants to be in director as opposed to actor stance when it comes to his character.
You and the GM, however, don’t.
(Personal note: The way you write about his GF also sounds quite dismissive – this is how women tend to get into the hobby unfortunately as it’s still somewhat stigmatized – please try and be welcoming, play off of her character etc. How about f.e. the GM contacts her and asks her about an idea for her character he had, like meeting whoever-is-in-her-backstory and then X happens and there are trials her character has to solve like a puzzle bit and other bit and then she learns X about herself? No need to talk about specific puzzles, the theme of it is important. Not all players – especially again new ones to the hobby – have planned out milestones for their characters. I myself, in fact, would find that boring. So yeah this is not about planning stuff in minute detail, but sketching a rough mission that is just for her character to be the star in. A thing every GM should do, but with buy-in from the player!)
Can you salvage this?
Maybe. I think there are two ways to do this. Both of the options recquire talking to your problem player (one on one, GM and him) and figuring out what kind of consequences he wants. Give him examples like the ones I mention in point one. Does a giant kill you if he smashes you? Injures you on failed roll? Just dents your armor? Those choices affect the feel of the whole game and are super important.
- The easiest path for him, hardest for you and GM. Play the way he wants to. Reboot the game, look for a system that works with that style of play and go run into fantasyland. State consequences explicitly, use Fate / Fu / whatnot points as rewind options, basically empower the players to tell a fun story together. This is best done in narrative-focused games (again something like Fate, FU and whatnot).
It does, however, require a huge change of the GMs and your mindset. Play grand stuff like Star Wars or whatnot, if a roll is failed the GM should be all ‘oooh, how about the Giant moves just a hitch too fast, you think something has to be up with that, maybe he has enchanted items on him? How about he grabs you and smashes you against the wall, denting your armor a bit!’
I myself am not flexible enough to GM that kind of power-fantasy style, but many GMs are more flexible.
- Session 0. Hard on everyone and will definitely lead to more conflict. Maybe, again, add Fate or whatnot points to your D&D game in order to be able to rewind stuff and have FUN with the possibilities. Have immediate rather than long-running consequences. The points can be removed at some point (if he agrees to that) but think of them like training wheels that lead to less frustration for both him and you.
Talk about what he expects from a game, what he wants to have. Talk about what you understood from what he said and if that’s correct. Give example scenarios.
If he still blows up, remind him gently of what was agreed to and take a short break to eat, go for a walk, doesn’t matter. Give him time to, well, kind of grieve? Work through the disappointment and frustration, definitely.
With both options, it’s also really important for both of you to have less of a ‘this is how roleplay works’ outlook and to encourage him.
Positive reinforcement works so much better than constantly beating someone down. We as a society know this from training all kinds of animals, including small humans. Use that. Go ‘oh, that idea is so cool, let’s try it and rewind if it doesn’t work with a Fate point!’ Not: ‘Let me swoop in because I’m the only and bestest roleplayers here of all roleplayers, totally taking the limelight’ because that’s how some of these situations come across from his perspective.
Put yourself in his shoes. Would you like to be treated that away, always told you’re doing it wrong when you just want to have some fun with your friends?
If neither the GM & you nor him can adjust: Then yeah, break up the group. It’s a common nerdfallacy that friends have to do absolutely everything together. Do whatever is fun with the whole group together, not something that only adds conflict.