Hermione would be in shock at first. She'd be left feeling very alone, especially since Harry and Ron are her closest friends. But she would soon turn that shock into action. She could start reaching out to other allies she has made over the years at Hogwarts. Maybe she'd team up with Neville or Luna to try and find out what made Harry and Ron abandon her, and also to find a way to bring them back.
She could initially be angry. She might feel that all her efforts to help Harry and Ron were for nothing. However, she would quickly realize that she needs to figure out why they abandoned her. She would start investigating, using her intelligence and resourcefulness. She might look for clues in the library, or try to recall any strange behavior from Harry and Ron before they left her. And she would not stop until she got to the bottom of it.
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 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.
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.
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.
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.
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.
It could be that Ron and Hermione abandon Harry due to a prophecy that they misinterpret. They believe that by being with Harry, they will bring doom to the wizarding world. So they go into hiding. Harry, on the other hand, is determined to change the course of the prophecy. He embarks on a journey to find the truth behind it, all while feeling the pain of his friends' desertion. He has to fight against dark forces that are taking advantage of the situation.
In such a fanfiction, it might be that Harry and Ron are influenced by a powerful dark spell. This spell makes them see Hermione as a threat, so they abandon her. Hermione then has to find a way to break the spell's influence on them while also dealing with the loneliness and danger she's now in. She could seek help from other Hogwarts students or even some friendly magical creatures.
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.
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.
Hermione may also become more empathetic towards those who are considered outsiders, like Draco. Having been abandoned by her closest friends, she can understand what it's like to be alone. This could lead to her reaching out to Draco in a way she might not have before. She could see past his Slytherin exterior and try to understand his insecurities and motives, which in turn could change her own character as she becomes more open - minded.