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 And he could become very withdrawn, not wanting to talk to anyone, even his closest friends like Ron and Hermione.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.
There are those who, when caught cheating, will try to shift the blame. In a business deal, if one partner is caught cheating on the contract terms, they might try to say that the other party didn't communicate clearly. But in general, being caught cheating often leads to a loss of trust. Whether it's in a friendship, a relationship, or a professional setting, once caught cheating, it's very difficult to regain the respect and trust of others.
Their friends in Gryffindor might be quite shocked at first. Ron, being Harry's best friend, would probably be a bit angry and concerned. He'd lecture Harry about setting a bad example. Hermione would be more worried about the consequences, like if they got caught by the teachers. Neville might be a bit more understanding, having had his own experiences of growing and making mistakes. Luna would probably just smile and say something really off - the - wall like 'Oh, the drunk stars are making you do interesting things.'
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.
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.