Troll picks up player and throws them in the air like 50ft and the player-at 10 hp for context--is out of spell slots but they have the create bonfire cantrip.

Can they evade death completely by creating a immobile bonfire in the sky or are they royally screwed?

Edit: I just realized the description said on the ground.

Create Bonfire explicitly says the bonfire is created on the ground, so no, you can't create it in the sky. 

I'm not really clear on how it would help in this situation if you could, though. It's just creating a 5-foot cube filled with fire; I don't see how that would stop you from falling.

One of my players argued for it and i just went with it. I didn't read the exact spell description about it being on the ground so i feel like a bit of a idiot right now. Also my player had fire immunity so I just went with the fire solidifying because it was funny.

I mean, ultimately it's all about people having fun. Do what your table finds fun, as long as everyone's okay with going off-book for some things.