One possibility could be that she begs for the truth. In a lot of fanfics, when Hermione is betrayed, she might beg the person who betrayed her, perhaps Harry if the plot is set that way, to tell her why they did it. She's a character who values loyalty and knowledge, so being betrayed would be a huge shock. And her begging could be a way to try and make sense of the situation, to get back to the stability she once knew.
She could be begging for forgiveness on behalf of the betrayer. Maybe in the fanfic, she realizes that there are greater forces at play and she doesn't want the relationship to be completely broken. So she begs the wronged party (it could be other friends or the whole wizarding community in some cases) to forgive the betrayal. This shows her selfless side and her ability to look beyond the immediate hurt.
She might become more distrustful. After being betrayed, it's natural for her to have a hard time trusting others again.
Well, a plot could involve Hermione being influenced by a powerful dark magic item. This item warps her mind and makes her see Harry as a threat to the wizarding world. So, she betrays him by revealing his secrets to his enemies. Or perhaps she gets jealous of Harry's popularity and power and that leads to her betrayal in a moment of weakness.
Hermione might seek revenge. Given her intelligence and magical abilities, she could use her skills to get back at Harry and Ron for their betrayal. This could lead to a big rift in their friendship and even endanger their common goals in the wizarding world. She might also side with other characters who are against Harry and Ron, changing the whole dynamic of the story.
One possible plotline could be that Ron and Hermione are influenced by dark magic or a powerful enemy who manipulates them into betraying Harry. For example, they might be under the Imperius Curse. Another could be that they misunderstand Harry's actions or intentions, leading them to believe he has gone to the dark side, so they turn against him in an attempt to 'save' the wizarding world. Maybe they are jealous of Harry's fame and power growing too much, and this envy festers until they decide to betray him.
One possible fanfic could be that in Hogwarts, a new student who doesn't know the trio well sees Harry and Hermione constantly together in the library and during study sessions. The new student spreads the word that they are a couple. This leads to some awkward and funny situations. For example, Ron might be a bit jealous at first, thinking that Harry has somehow'stolen' Hermione's attention from him. But then they all realize it's just a misunderstanding and have a good laugh about it.
Well, there's a fanfiction titled 'A World Turned Upside Down' in this genre. In this story, the betrayal is not as straightforward as it seems at first. It shows how misinformation and outside forces play a role in creating the perception of betrayal between Harry and Hermione. It also has some great character development as they struggle to come to terms with what has happened.
Maybe a new dark wizard or witch emerges. This character is extremely powerful and manipulative. They target Ron and Hermione because they are Harry's closest friends. By turning them against Harry, this antagonist hopes to isolate Harry and make him easier to defeat. The new antagonist could have some unique magical abilities that allow them to control the minds of others or create very convincing false scenarios.
He might be initially heartbroken. Harry has deep feelings for Hermione, so the rejection could come as a shock. He could withdraw into himself for a while, feeling sad and confused about why she doesn't feel the same way.
They could become even closer friends. After going through the war together, they have a deep bond. Maybe they start to rely on each other more than ever, sharing every little detail of their lives.
I'm not sure specifically as there could be many different interpretations in fanfiction. It might be about a situation where The Order, perhaps in the Harry Potter universe, betrays Hermione in some way, like leaving her in a dangerous situation, or not believing her when she has crucial information.