I'm not sure specifically if there is a fanfiction titled exactly like that, but in the Harry Potter fanfiction community, all kinds of relationship - based plot twists are explored. It could be that someone has written a story where Cho cheats on Harry. You can try searching on popular fanfiction websites like Archive of Our Own or FanFiction.net to find out.
In this fanfiction about the Yule Ball, there are many possibilities for Harry and Cho. They could be initially hesitant around each other. However, the Yule Ball is a special event full of magic and wonder. Maybe they find a quiet corner to talk, away from the noise and the crowd. During their conversation, Cho might open up about her feelings for Cedric, which could lead to Harry comforting her in a new and deeper way, strengthening their bond.
Hermione Granger could be one of the characters. Since she is Harry's close friend, it would be interesting to see how she fits into the harem dynamic, whether she's supportive or has some mixed feelings about it.
One common theme is unrequited love. In many fanfictions, Cho's feelings for Harry are often more intense than he initially realizes. Another theme is dealing with grief. Given their experiences in the wizarding world, they often have to cope with the loss of friends together.
Love and heartbreak are common themes. Their relationship in the original series was complex and full of unspoken feelings, so in fanfiction, authors often explore those emotions further. Another theme is self - discovery. As both characters grow, fanfiction may show how they discover more about themselves through their relationship with each other.
Harry is the most important character interacting with Yandere Cho. But we can't forget about Luna Lovegood. Luna might notice Cho's abnormal behavior and try to warn Harry. Also, Cedric Diggory's memory could play a role. Since Cho was Cedric's girlfriend before, his memory might haunt her and fuel her yandere tendencies, and this could be noticed by characters like Harry when they interact with her.
Cho's comfort can make Harry more open. Usually, Harry is quite reserved about his feelings. But when Cho comforts him, he starts to share more. For example, he might open up about his fears regarding Voldemort.
Another theme is the struggle with past traumas. Given the events at Hogwarts, especially the death of Cedric Diggory which deeply affected Cho, fanfictions often explore how they both deal with their past experiences while trying to build a relationship. It's about healing and moving forward together.