Read some of the comments at the link below, there’s a lot of circumstantial proof this letter is real.
http://www.wtvm.com/story/25971032/military-themed-sticker-draws-criticism-angry-letter
https://www.thefederalistpapers.org/us/urgent-update-military-fiance-returns-to-her-car-to-find-a-note-on-her-windshield-spewing-hate
I’m betting that she wrote it herself.