Connie Poirier killed Elly Patterson in the For Better Or For Worse fanfic The New Retcons because Elly didn't return her feelings. She later played it off as an Accidental Murder, claiming John was the target rather than have everyone know it was this trope.
If she just didn't return her feelings, this doesn't go here as there was no cheating involved.
Woman Kills Woman
If she just didn't return her feelings, this doesn't go here as there was no cheating involved.