Okay but this is a very easy, if embarrassing, conversation to have.
“I know the author of the work you are plagiarizing personally, and I have already contacted them to verify that you did not have permission to reference/quote, and that their work was uploaded long before your work was finished.”
And if the kid asks how you knew it was plagiarized, “I know the author personally”, or why you recognized the work, “it was a personal favor/ is not of your business as my student”, and literally every other question can be rebuffed in the same way.
Like, it seems like none of you have ever had to do professional level bullshitery and it shows.
Sure, its embarrassing to even alude to the fact you read fic, but just remember, the kid not only read your fic, but thought it was good enough to try and steal too.
“I was directly involved in the editorial process.”
ALT