Bell could be deeply hurt by the betrayal. She might start to have nightmares about it. As the story progresses, she could find the strength within herself to confront her so - called friends. She could demand an explanation and then decide whether to forgive them or cut them out of her life completely. She might also become more cautious in making new friends in the future, always being on guard against potential betrayal.
He probably feels extremely hurt and confused. Betrayal by friends is a very painful experience, and Bell might question his own judgment in trusting them in the first place.
He might become very withdrawn. Bell could start to isolate himself from others, especially those associated with Hestia. He may no longer trust as easily as he did before.
Sakura could become very angry. She has been let down by those she trusted, and anger is a natural response. She might lash out at her friends or distance herself from them immediately. For example, she could say some harsh words to them in the heat of the moment.
In a 'Twilight Betrayed by Her Friends' fanfiction, it could be that Twilight's friends become jealous of her powers. They feel overshadowed by her abilities and knowledge. So they start to undermine her at every turn. For example, they might ruin her chances of getting a special magical promotion or getting into an exclusive magical study group. Twilight is heartbroken at first but then decides to prove herself without their support and in the process, she might find new friends who truly appreciate her for who she is.
One possible plot could be that Hermione's friends start believing false rumors about her. For example, they might think she's been stealing potions ingredients for personal gain when in fact she was framed. As a result, they shun her and exclude her from their group activities. This makes Hermione feel isolated and betrayed, and she has to find a way to clear her name and either regain her friends' trust or find new allies.
In a fanfic, the betrayal might also be a misunderstanding. Her friends could be misled by false information about Sakura. For example, an enemy might spread rumors that Sakura is planning to betray them first, causing her friends to turn against her without really knowing the truth.
It could be a character from a rival group. They see Bell Cranel as a threat to their plans and so they betray him by setting him up in a dangerous situation.
It could be an outsider who manipulates Twilight's friends. This character might be envious of Twilight's popularity or power in the magical realm. They trick Twilight's friends into believing false things about her, making the friends turn against her.