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.
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.
Naruto's power might also be affected. If he was betrayed, he could lose the will to fight or train. He might even start to doubt his own purpose. This could lead to a decline in his abilities, which in turn could put the village at risk if he was its main protector.
He might become very withdrawn.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.
In such a fanfiction, perhaps Harry's friends are jealous of his newfound powers or popularity within the wizarding world. So they spread rumors about him being untrustworthy. As a result, Harry is shunned by many in the community. He has to embark on a solo journey to prove his innocence and in the process discovers who his true friends really are.
In a fanfiction where Naruto is betrayed by his friends, it could start with jealousy. Suppose Naruto gets a great power or a special recognition. His friends, feeling overshadowed, might be tricked by an evil force into thinking Naruto is using them. So they betray him, which makes Naruto realize that he has to rely on his own strength and find new allies who truly understand him.
Sometimes in fanfictions, character development takes a dark turn. The friends might have their own personal goals that conflict with Naruto's. They could see Naruto as an obstacle to achieving those goals. Or perhaps there was a long - hidden secret that came to light, which made them change their attitude towards Naruto, leading to the so - called betrayal.
The Ministry of Magic could also be the main antagonist in this case. If the Ministry is corrupted or under the influence of dark forces, they could manipulate Harry's friends. They might convince his friends that Harry is a danger to the wizarding world order, and his friends, being naive or misinformed, end up betraying him.