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.
Ash could respond in different ways. He might try to confront his friends immediately to get an explanation. If the betrayal is really bad, he could cut them off completely and focus on his own goals, like becoming a better Pokémon trainer. He would also probably have some internal struggles, as he has memories and experiences with these friends that he can't just forget.
He might be extremely hurt. Harry could go into a state of shock and disbelief at first. He may isolate himself, not wanting to trust anyone else for a while.
He might be initially shocked. Harry has always trusted his friends, so when the betrayal happens, it would be like a bolt from the blue. He could stand there speechless for a moment, unable to believe what has just occurred.
She could be angry. Anger is a common reaction to betrayal.
He might become very withdrawn.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.
Vernon could be quite curious. He'll be eager to read what others have come up with. There might be new ideas, different takes on his character or his world that he hadn't thought of before. So he's probably excited to explore these new interpretations.
In such fanfiction, it can be a complex situation. Maybe Ash's friends were misled or had their own hidden motives.
He would be heartbroken. Naruto values friendship above almost everything. At first, he might not believe it and try to reason with his friends.
At first, Naruto might be really confused. He's used to thinking of his friends as his support system, so when they turn on him, he doesn't understand why. He'll probably try to talk to them, to ask what he did wrong. If they don't listen, he could become really sad and withdrawn. Maybe he'll start training even harder, using the pain of being rejected by his friends as motivation to become stronger.
Sonic usually feels a great deal of shock and confusion at first. He can't believe that his friends would turn on him. Then he might feel anger, especially if he discovers that they were misled or acting out of unjust reasons.