Harry could also go into a state of shock and disbelief. He loves Ginny deeply, and to find out she's cheating would shatter his world. He might start to question everything about their relationship, like all the memories they've shared together. And he could become very withdrawn, not wanting to talk to anyone, even his closest friends like Ron and Hermione. He would need time to process his emotions and figure out what to do next.
At the very beginning, he might act impulsively. For example, he could immediately break up with Ginny without even listening to her explanation. But as time passes, he might start to have second thoughts. He might wonder if there were signs he missed before, or if there's a chance that things could be different. This could lead to a complex emotional journey for Harry as he tries to come to terms with what he's discovered.
In such a fanfic, perhaps Ginny has been feeling neglected by Harry due to his constant focus on fighting dark forces or some other important matters. So she starts to seek comfort in someone else. When Harry finds out, he first feels angry and betrayed. But then he realizes that he also had a part to play in their relationship going sour. He could then try to win Ginny back or decide to move on, depending on how the author wants to develop the story.
Harry might immediately rush to her side and comfort her. He could put his arm around her and ask her what's wrong in a gentle voice.
Ginny's reaction could also involve a sense of helplessness at first. But then she'd quickly gather herself and use her resources. For instance, she might call on her friends and family for help. She'd be by Harry's side constantly, using her presence to comfort him and also using her knowledge of magic to try and ease his pain.
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?'
Ginny might be overjoyed. She could blush deeply and then give Harry a big hug, telling him she feels the same way.
She might blush. Since Ginny has feelings for Harry, his stare could make her feel self - conscious and her cheeks turn red.
He could be initially shocked. Harry, who has trusted Ginny deeply, would find it hard to believe at first. He might go through a period of denial, trying to find excuses for Ginny's actions.
Alternatively, Ginny could be very mature about it. She might respect Harry's decision, even though it hurts. She could choose to step back and give Harry the space he seems to need while secretly hoping that he will change his mind in the future.
On the other hand, Ginny might secretly be a bit flattered that Harry is so possessive over her. But still, she would probably try to stop the situation from getting worse and make Harry understand that he can't just go around punching people because of her.
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.