First of all, his friends would likely be very curious. Since Naruto has had a rather eventful childhood, the fanfiction could contain a lot of personal elements. They might be amused by his childish handwriting or the simple way he used to express his ideas. Some might even be touched if the fanfiction reveals Naruto's hidden insecurities or hopes from back then. It could also lead to his friends having a new perspective on Naruto, seeing a side of him that they didn't know before.
Sasuke, who has his own complex relationship with Naruto, would probably be in denial at first. He's seen Naruto as this strong - willed person, and the idea of him having schizophrenia would be hard to accept. But as time goes on, he would become more protective. He might train Naruto in ways to control his mind during the episodes, using his own experiences of mental discipline. As for Kakashi, he would be empathetic. He has his own traumas, and he would do his best to support Naruto, maybe sharing his own stories of dealing with difficult mental states.
There are fanfictions where Naruto's power is shown through his ability to connect with the spirits of past Hokages. With their combined wisdom and his own strength, he's able to perform a super - powerful jutsu that not only protects Konoha but also restores the balance of the ninja world. He uses this new - found power to face an ancient evil that has been awakened.
In some fanfictions, Naruto protects his friends by using his new and improved jutsu. For example, he might develop a more powerful Rasengan to fend off stronger enemies. He also uses his quick thinking and ability to sense danger, which he has honed over time. His unwavering loyalty drives him to put himself in harm's way without hesitation, always being the first to jump into a fight when his friends are threatened.
In fanfiction, Naruto jumping in front of his friends could be a heroic act. Maybe he's protecting them from an incoming attack. It shows his selfless nature, always ready to sacrifice for those he cares about.
Some friends might feel betrayed. If they are very clan - oriented, they could see Naruto's leaving as a sign that he doesn't value the clan and by extension, their friendship. However, his true friends like Sasuke or Sakura might understand his reasons. Sasuke, who has his own issues with his clan, might be more empathetic. Sakura, who has seen Naruto's determination, could support his decision in the long run.
In some fanfictions, Naruto's pain might manifest through his actions. For example, he could become withdrawn and isolate himself from his friends. This is because Naruto has always been a social person, so any deviation from his normal behavior could be a sign of his internal pain.
One possible cause could be his sense of failure. Naruto always wants to protect his friends and village, so if he fails in a mission or is unable to save someone, it can cause him great pain.
His relationship with Sasuke might become more complex. Sasuke, always the rival, might see Naruto's new overlord status as a challenge to his own power and ideals. They could have more intense ideological battles.