Ash would probably be devastated. He would feel like his world has been turned upside down. At first, he might not believe it and think it's all a big mistake. As the reality sinks in, he could start to isolate himself from others, even from his Pokemon for a while. However, his strong will would eventually kick in. He would use this as motivation to become stronger, not just in his Pokemon battles but also in his ability to face challenges alone, without relying on his so - called friends.
Jealousy could be one reason. Maybe his friends are jealous of his achievements as a Pokémon trainer. Another reason could be that they are influenced by some external forces, like an evil organization that convinces them to betray Ash.
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.
Ash would probably be initially shocked. He has always trusted his friends, so the idea of betrayal would be hard to swallow. He might try to talk to them, to find out why they are doing this.
One possible reason could be jealousy. Maybe Ash has achieved more in his Pokemon journey than his friends, and they can't handle it. Another reason might be some misunderstanding. For example, an enemy could trick Ash's friends into thinking Ash has done something bad. Or perhaps there's some sort of power struggle. If there's a powerful item or a rare Pokemon involved, and Ash is the main contender, his friends might be tempted to betray him for their own gain.
One possible reason could be jealousy. Maybe his friends are jealous of Ash's success in his Pokemon journey. For example, if Ash wins a big tournament and gets all the fame, his friends might feel left out and betrayed. Another reason could be a misunderstanding. Ash might do something that his friends misinterpret. Say he makes a deal with an enemy in order to save his Pokemon, but his friends don't understand the situation and think he's turned on them.
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.
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.