The University of Central Florida student told BBC Newsbeat he was dating his ex for eight months, and "Four months in she started hiding her phone and I heard she had code names for guys in her contacts list."
When the cheater wrote a long apology letter, Lutz took the opportunity to give her a free lesson in writing and editing, offering some solid critiques.
Professor Lutz critiques the opening for being overly long with "lots of repetition," and even points out the lack of proper indentation at the beginning for the paragraph.
"Strong statement. No supporting details to support your hypothesis," he writes. Damn.
Ever the editor, Lutz points out that the second page could use more details on the wrongdoings, and that "loose" is not the same as "lose."
As the essay keeps rambling, Lutz criticizes her "lackadaisical" handwriting and overuse of "useless fillers."
In the final page, the author writes, "I love you," which Lutz corrects with a question mark on the end.
He ultimately give his ex's d-fense a D-.
"Long intro, short conclusion, strong hypothesis but nothing to back it up. Details are important," he writes.
"Need to stop contradicting your own story and pick a side. While this gesture is appreciated, I would prefer details over statements. Revision for half credit will be accepted."
The internet then joined in to collaborate on the copyediting,
While the cheater failed him, Lutz got to fail her back. He even scored a new date, who hopefully has better grammar skills. And won't cheat, of course.