He'd probably be furious at first. He has a strong sense of loyalty and would feel deeply hurt by Ginny's betrayal.
In such a fanfiction, Harry might first feel shocked and betrayed. He could distance himself from Ginny immediately. Then, he might focus more on his friends and his own personal growth, perhaps delving deeper into his magical studies or his role in the wizarding world. He might also struggle with trust issues in future relationships.
Sirius might react with a mix of surprise and protectiveness. He would be like an older brother or father figure. He'd make sure they were safe and not doing anything too risky.
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 And he could become very withdrawn, not wanting to talk to anyone, even his closest friends like Ron and Hermione.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.
Ginny might be extremely embarrassed. She would probably blush deeply and try to hide behind Harry at first.
Ginny would likely be protective. She loves Harry and knows about his difficult past with the Dursleys. So she would probably stand up for him right away, not tolerating any of their mean remarks.
Harry might blush and stammer. He could say something like, 'Sirius, it's not like that.' and then quickly try to change the subject.
Ginny might be in disbelief for a moment. She has always seen Harry as this heroic figure and now he's asking her to the ball. But then her natural confidence would take over and she would accept with enthusiasm. She would also tease Harry a little bit about taking so long to ask her, adding a bit of humor to the situation.
Harry would probably be heartbroken. He trusts Ginny deeply, so her betrayal would be a huge shock. He might initially be in denial, not wanting to believe that Ginny could do such a thing.
He might be amused. Fanfiction can be quite creative and sometimes funny, so he could just laugh it off and think it's an interesting take on the characters.
Their friends would be in shock. Ron, being Harry's best friend and Ginny's brother, would be torn between loyalty to both. Hermione would probably be disappointed in Ginny and very supportive of Harry. The Weasley family as a whole would be divided. Mrs. Weasley, who loves Harry like a son, would be heartbroken and angry at Ginny. Their other friends at Hogwarts or in the Order of the Phoenix would likely gossip and have mixed feelings. Some might pity Ginny for the difficult situation she got herself into, but also condemn her for cheating on Harry.