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.
One possible plot could be that Naruto discovers his friends have been secretly working with an enemy. They might have been promised power or something valuable in return for betraying Naruto. Maybe they spread false rumors about Naruto, turning the village against him.
One common theme could be Ash's feelings of isolation and despair. After being betrayed, he would likely feel very alone and question his past relationships.
I'm not sure of specific fanfics on this topic offhand. You could try searching on popular fanfic platforms like Archive of Our Own. Just type in the keywords 'ash gets betrayed by most of his friends' in the search bar and browse through the results.
One possible reason could be jealousy. Maybe his friends become envious of Harry's growing powers or popularity. For example, if Harry is suddenly getting all the attention in their magical community, his friends might feel left out and start to resent him, leading to betrayal.
Naruto would likely become more distrustful at first. He's always been so open and trusting of his friends, so this betrayal would shatter that. He might become more withdrawn, constantly second - guessing people's motives.
At first, Harry would be in disbelief. He would think it was some sort of joke or misunderstanding. But as the reality sets in, he would become a bit of a loner. He'd still have his sense of right and wrong, so he might continue to fight against the dark forces on his own, all the while hoping that his friends would come to their senses and realize they made a huge mistake in betraying him.
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 some 'harry gets betrayed fanfic', it could be Dumbledore who betrays Harry in a way. Maybe Dumbledore has been using Harry all along for a greater plan without fully disclosing it to him. Harry would feel extremely hurt and betrayed as Dumbledore was like a mentor to him. This would lead to a series of events where Harry questions everything he has known and tries to find his own path.
Well, usually in such fanfictions, Harry might face a lot of emotional turmoil and have to find his way alone to overcome the betrayal and the challenges that come with it.