The trickster set up an online account posing as the the QI host judge and messaged show contestant Ruby Bhogal.
He set up video recording for his phone screen and captured the non-public direct messages between himself and what appears to be Ruby's genuine account.
They chatted about vegan week and the Danish-themed episode - before Ruby seemingly revealed she was "very happy" a particular baker had won.
She was tricked by a 26-year-old man from Swansea who said he did it because he was "bored".
After setting up the account, he started messaging Ruby from the show on Thursday (October 11) about last week's episode.
The fake Twitter account of Sandi mimicked the host's genuine account, using her photo, but the handle was spelled slightly differently.
Fake 'Sandi', said: "Hi Ruby, I bet you were watching yesterday's show through your fingers! Hope you're doing well x."
Ruby replied: "Sandi! Lovely to hear from you.
"Oh - let's not talk about that. Was just as traumatic as I remembered. You and Noel were absolute angels to me though, still very thankful for that! How are you? x"
The pair talk about vegan week, the Danish episode, the spin off An Extra Slice, and fans asking for selfies.
Ruby then went on to reveal she is "really happy" with the eventual winner of the show.
The anonymous online prankster says he was simply "bored" when he decided to trick Ruby.
He said: "It's pretty easy to do really.
"I recorded the screen on my phone of the message.
"It started with me saying I bet you watching yesterday's show through your fingers."
Leeds' Kim Joy lives in the city with her partner. She was born in Belgium to an English father and Malaysian–Chinese mother.
She grew up in London, went to University in Bristol and is now living in Leeds, and has made the semi-final of the competition.
The final of Great British Bake Off will take place in less than two weeks, on Tuesday, October 30.