In the fanfiction world, miscommunication often plays a big role. Perhaps Harry was involved in some secret mission or task that he couldn't tell Ginny about right away. But Ginny, not knowing the real situation, might misinterpret his secrecy as cheating. For instance, he might be sneaking around to protect Ginny from a potential threat, but she doesn't see it that way.
Well, it could be that Harry has started acting really distant from Ginny. He might be canceling their dates frequently or not being as affectionate as he used to be. In the fanfiction, Ginny might also find some strange gifts or notes that she suspects are from another girl to Harry, which makes her think he's cheating.
She might directly ask him about her suspicions. Just come out and say 'Harry, I think you're cheating on me. I've noticed you acting strange lately.'
One possible reason could be miscommunication. Maybe Ginny feels neglected by Harry because he's so caught up in his own heroic deeds and doesn't pay enough attention to her. So she might seek comfort or excitement elsewhere.
Maybe she saw Harry spending a lot of time with another girl without proper explanation. In a fanfic, it could be that Harry is often with a new female character in his study or on some secret missions, and Ginny misinterprets this as something more than friendship.
He could start by being completely honest with Ginny. Sit her down and tell her everything he has been doing, leaving no details out. For example, if he was working on a project with another character, he should explain the nature of the project, who else was involved, and why he had to keep it somewhat of a secret.
In fanfiction, anything can happen. However, if Ginny cheats on Harry, it would likely shatter the trust in their relationship. It could lead to a downward spiral in the story, with Harry feeling betrayed and perhaps seeking revenge or just distancing himself. This could also change the dynamic among their friends and family, as everyone would be caught in the middle of this drama.
She might confront Harry directly. In the heat of the moment, she could burst out with her suspicions and demand an explanation. For example, she might say, 'Harry, I've noticed you've been acting strange around that girl. Are you cheating on me?'
In the fanfic, it could be that there are signs that Beckett is hiding something. Perhaps he has unexplained absences. He might say he's at work but Alexis can't reach him during those times. Also, he could be overly protective of his phone or personal space, which makes Alexis suspect that he's cheating.
I really hope there isn't such a fanfiction. Ginny and Harry have a strong and loving relationship in the original story, and creating a fanfiction with such a negative and unfaithful plot goes against the essence of their relationship.
One possible reason could be a growing apart due to their different career paths. Harry, being deeply involved in the wizarding world's security and Ginny having her own Quidditch career. Another reason might be the stress of living in the public eye. Their relationship may not be able to withstand the constant scrutiny and pressure from the wizarding community.