It's hard to imagine a better learning opportunity than this. And anyway, do we want to destroy her life, or help her learn a very important lesson?
Be creative, be effective. For example, in addition to giving her an incomplete and having her re-do the assignment, you could reduce her final grade by one letter (i.e. from B to C), and you could have her write a separate research paper describing three famous plagiarism scandals of her choice.
To quote a classic story:
At a certain school in [your region here] some girls were beginning to use lipstick, putting it on in the school bathroom. After they put on their lipstick, they pressed their lips on the mirror, leaving dozens of little lip prints.
Every night, the maintenance man removed the prints, but the next day,
the girls put them right back. Finally, the principal decided that
something had to be done. She called all the girls into the bathroom
along with the maintenance man. She explained that all these lip
prints were causing a major problem for the custodian, who had to
clean the mirrors every night. This inspired yawns from all the little
To demonstrate how difficult it was to clean the mirrors, she asked
the maintenance man to show the girls how much effort was required. He
took out a long-handled squeegee, dipped it in the toilet, and cleaned
the mirror with it. Since then there have been no lip prints on the
There are teachers… and then there are educators.
As a graduate student instructor, I encountered these problems occasionally. In one case, as an example, of a student who had not done his own programming, the end result was that he had to take the class again in the summer. He learned to write simple programs, and he learned to hand in his own work, not someone else's. And he got a Bachelor's degree.