[Kusayarou] 100-nichigo ni Netorareru Kanojo ~Netorareru made Ato 60-nichi~ | The Girlfriend Who Was Cucked After 100 Days - 60 Days Until Cucked [English] [Throwaway5432]
This one ended a long time ago didn't it? If I remember correctly she obviously totally sluts out and shows up at school on the 100th day blond and dressed super slutty and making out with the blond, and of course that's how the boyfriend finally figures it out. I couldn't find the full version when I just looked through the artist's gallery.