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.
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 He might become more withdrawn, constantly second - guessing people's motives.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, Naruto would be in disbelief. He has always trusted his friends wholeheartedly, so this would be a huge blow. He might then feel a deep sense of anger and sadness. However, Naruto being Naruto, he would likely also start to look for ways to prove his innocence or the misunderstanding. He might go on a journey of self - discovery to figure out what went wrong and how he can win back his friends' trust, all while dealing with the pain of the betrayal.
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.
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.
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.
In most 'Naruto gets sick' fanfics, his friends are very concerned. For example, Sakura will immediately start using her medical skills to diagnose and treat him. Sasuke, although he may not show it outwardly as much, will stay by Naruto's side and keep a watchful eye. Shikamaru might come up with plans to make Naruto more comfortable during his illness. And Hinata, she would be really worried and would probably bring Naruto some home - cooked food to help him regain his strength.
His friends would most likely rally around him. Kiba might use his dog Akamaru to scout the area and make sure there are no other immediate threats. Hinata would be really worried and would try to use her Byakugan to see if there are any hidden enemies or if Naruto has any internal injuries. Choji would offer some of his food to Naruto to give him strength, as his clan's food can have special effects on a ninja's body.
It's a very strange concept for a fanfic. Naruto is known for his strong bonds with his friends. In the Naruto universe, friendship is a major theme. So for him to get revenge on his friends would mean something really bad must have happened. It could be an interesting fanfic if the writer can come up with a really good backstory to justify it. For example, if his friends were somehow mind - controlled and did things that hurt Naruto deeply. But overall, it's not something we'd expect from the Naruto we know.