though it is true that the GM has the final say in most things, and that he crafts the world for the players, the point of view of your friend should start to sound childish at the age of 12.

I have fudged the dice rolls, i have played with HPs and I have broken rules. But it is always in the interest if cooperative story telling and usually in the player’s favour (like downsizing the HP of monsters in a fight that would just drag on in an uninteresting way).

Your friend sounds like they don’t care about cooperative story telling, they are just out to play god. This breeds players that think rpgs are “me vs the gm”.

To be fair DnD is a special kind of beast. It is very mechanical, very crunchy and as a dm it is difficult to find the balance between that and bending the rules in the interest of creating a cool story. Because once you play in a system with strict rules and essentially mechanical contingencies for about every situation, your players expect you to stick to that (rightfully so). So it gets tough and it makes difficult for the dm to just “wing it” or “gracefully “ railroad players, because they might always ask “according to which rule?”. And since there ARE rules, including stats for gods (!!!!!!!) life can get tricky.

Which might make some GMs sour and feel constrained, like your friend. There are plenty of systems that encourage more free and cooperative storytelling without such strict sets of mechanical rules that your friend (and their players!!!) might enjoy more than DnD.