Harry would be extremely angry and immediately rush to her rescue. He has a strong sense of loyalty towards Hermione and would not tolerate her being in such a dangerous situation. Ron, on the other hand, would also be very worried. He might start panicking a bit at first but then quickly gather his courage and follow Harry's lead in trying to free Hermione.
One possible reason could be that she has been captured by some dark forces who want to extract information from her. They might think she has valuable knowledge about the Order of the Phoenix or some powerful magic. Another reason could be that it's a part of a dark ritual. Some evil wizards or witches might believe that her magic or her connection to certain powerful magical artifacts can be harnessed in this way for their own malicious purposes.
Hermione would probably be deeply hurt at first. She trusts Harry and Ron so much, and being abandoned by them would be a huge blow. But she's also very determined. So, she might throw herself into her studies even more, using her knowledge as a way to cope and also to find a solution to the situation.
At first, Harry might be confused. He would try to figure out what went wrong. He might search his memory for any signs that could have led to this betrayal. Then, he would likely experience a range of emotions from sadness to rage. Depending on the nature of the betrayal, he could either try to win them back by showing them the truth or decide that they are no longer worthy of his friendship and focus on his own path, maybe even leaving the wizarding world for a while to clear his head.
Harry's reaction would depend on the nature of the fanfiction. If it's a cute, romantic fanfiction about Ron and Hermione, he might smile to himself and think it's kind of sweet. However, if it's something that distorts their characters or has a negative slant, he could get defensive. He might even try to find out who wrote it and why they would write such a thing about his friends.
At first, Ron might be in disbelief. He might think it's some kind of joke. But when he realizes it's real, he could be a mix of hurt and confused. He might avoid Harry and Hermione for a while, but then, as he sees how happy they are at the ball, he might start to accept it and try to be happy for them too.
Perhaps she tries to find them right away. Using her intelligence and resourcefulness, she starts to search for any clues as to where they might have gone. She could ask other friends, like Neville or Luna, if they have seen Harry and Ron. And she might be determined to bring them back, not willing to give up on their friendship easily.
At the beginning, Ron would likely be in shock. He had always thought he and Hermione had a special bond. After the shock, he might become a bit bitter. However, his friends, like Harry, would probably try to cheer him up and help him see that there are other things in life. Eventually, Ron might find a new purpose in his life, whether it's in his magical studies or in exploring new parts of the wizarding world, and try to move on from Hermione.
Ron would be devastated. He might initially be at a loss for what to do. But then his Gryffindor courage would kick in. He'd start researching every possible cure or treatment for her injuries. He'd be the one to comfort Harry and the others while also being the most vocal in demanding justice for Hermione. His relationship with Hermione, whether you see it as just friends or something more, would make him act in a very passionate and protective way.
At first, Harry could be heartbroken. He would wonder what he did wrong to make them leave. He might try to find them and reason with them. However, if he can't reach them, he would likely turn that pain into a driving force. He would focus on his goals, like fighting against Voldemort or protecting the wizarding world, even more fiercely, because he now has to prove himself not only to the world but also to his former friends.
Another way she could react is by distancing herself completely. She would stop trying to be part of their group and start creating her own identity at Hogwarts. She might explore parts of the castle she's never been to before, find new study nooks, and interact with students she's never really talked to, all while trying to forget about Ron and Harry's cold treatment.