Well, [MENTION=9037]Elf Witch[/MENTION], let me ask you this. How was your campaign, that lasted two sessions, improved by the DM saying no?

Yes, because players are children and need structure and guidance in order to learn how to properly behave. It is pretty telling to see the characterisations of players here - immature, whiney, self entitled children given to tantrums if they don't get their way.
But, apparently, I'm a bad person for being negative about DM's by saying that maybe, sometimes, DM's should unclench a little bit and give the player what the player wants so long as it isn't going to invalidate large swaths of the campaign.
See, maybe if the DM wasn't so worried about micromanaging her campaign, and trusted that the players weren't out to ruin the game, and allowed the players to take what they wanted, she wouldn't have been burned twice. IOW, treat the players like responsible adults rather than spoiled children and maybe, just maybe, the players will act like responsible adults.
But, that's just me. I treat my players like responsible adults and put the responsibility for the game going well squarely on their shoulders. If the game goes wahoonie shaped, everyone at the table knows it's (at least in large part) their own fault. And guess what? I haven't had a game go wahoonie shaped in years. Once I actually took the plunge and treated players as equals, my game got better. It got a LOT better.
If you keep having the same experiences, perhaps making different decisions might, just might, be the solution.
It wasn't ...
But now we have a player who tried to DM twice and was burned twice and now won't DM again so you tell me how the game is served by driving DMs out of it?
If the game wasn't improved by the DM saying no, then the question is irrelevant.
Really? You would rather see budding DMs driven out of the game by bad behaviour than allowing them to find their feet and grow?
It sounds to me that what the game would have been improved by is the DM saying no and then not inviting the player to play. I wasn't there, I don't know, but it doesn't sound to me like poor DMing is the problem here. It sounds like an entitled player trying to take their ball and go home.If the game wasn't improved by the DM saying no, then the question is irrelevant.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.