At first, his friends would be shocked. Hermione might think it's some kind of dark magic at first glance. But as Harry shows them what he can do, she would start to analyze it scientifically. Ron would be more awed. He'd be like a kid seeing a new cool trick. They would both support Harry in exploring this new ability, but also be a bit cautious as they don't fully understand where it comes from or what the implications are in their wizarding world.
He could be shocked at first. Imagine finding out that your whole life has been based on half - truths. He might need some time alone to process it all.
He might be very sad at first. Harry has always valued his friends, so being forgotten by them would be a huge blow.
Their friends from the Order of the Phoenix would be overjoyed and offer protection. Neville might give them some special plants that are good for a magical household with a baby. Luna would share her unique and sometimes strange but helpful insights about the baby's future.
The fanfiction might be about Naruto accidentally stumbling upon a ancient scroll that contains the secrets of bending. He decides to learn it out of curiosity. He begins with fire bending, which is a challenge as it's very different from his chakra - based fire techniques. But with his ninja training and unwavering will, he manages to figure out how to control the flames in the new way of bending.
In this type of fanfiction, the relationship between Harry and Snape can be re - imagined. Snape was Harry's teacher for Occlumency in the books, but often their interactions were tense. Fanfic writers may change this dynamic. They could have Snape be more patient or Harry be more receptive, which would lead to a different kind of mentorship and a new understanding between them.
His friends would be quite shocked. Neville might be a bit scared at first, thinking it's some new dark magic. But then they'd all come together. Ginny would probably be the one to try and comfort 'cat - Harry' by giving him some fish or milk, thinking that's what cats like. And they'd all be determined to get him back to normal as soon as possible.
Naruto's friends in the Hidden Leaf would be really confused if Harry showed up. They'd wonder why he doesn't know basic ninja stuff. Harry's friends at Hogwarts would be equally baffled by Naruto. Hermione might try to study Naruto like he's a new magical creature. Ron would probably be a bit more hesitant around Naruto at first.
Luna would likely have a very unique reaction. She might say that the fanfiction is just another form of magic in the world of imagination. She could tell Harry that he should embrace it in a way, as it shows how much people are interested in his story, even if it is a bit mortifying at first. But she would also respect if he was really bothered by it.
Ron would probably be both shocked and a bit awestruck at first. He might make some typical Ron - like jokes to ease the tension, like 'Blimey, Harry, you're a dragon! At least you'll be good for scaring off Snape now!'. But deep down, he'd be worried and determined to help Harry get back to normal.
His friends would likely be devastated at first. Hermione, for instance, might throw herself into research to find out what really happened, believing there has to be a way to bring him back. Ron would be in a state of shock and anger, feeling like he failed to protect Harry. They would probably also start to question their own actions and whether they could have done something differently to prevent Harry's 'death'.