Some of his friends may try to find him immediately. They would be worried about his safety and want to know why he left. They might search all the places they think he could be, asking other people if they've seen him.
They could be initially shocked. They might not have expected Harry to leave at all. Some friends could be angry, feeling betrayed that he left without proper explanation.
Some friends might be angry at first. They could feel abandoned without really understanding Harry's motives. For instance, Ron might be frustrated and feel that Harry is being selfish by leaving without a proper explanation.
His friends might be shocked at first, but then start to really enjoy reading it and discussing the ideas in it.
His friends would be very worried. Annabeth would immediately start looking for solutions. She'd be using her strategic mind to figure out what kind of poison it is and how to get an antidote. Grover would be freaking out a bit but still try to be useful, like using his sense of smell to detect any strange scents around Percy that could give a clue about the poison.
Some might be excited. They've probably heard a lot about Percy from Annabeth, so they are eager to meet him in person.
His family could be initially shocked and heartbroken. For example, his wife might be left in a state of disbelief, wondering why he would leave without much explanation.
He might be extremely hurt. Harry could go into a state of shock and disbelief at first. He may isolate himself, not wanting to trust anyone else for a while.
The goblins could be indifferent initially. They have their own affairs and may not care much about a young wizard. But when Harry mentions something that interests them, like a possible treasure discovery or a threat to Gringotts, they start to pay more attention and engage in a more in - depth conversation with him.
The younger generation of friends like Boruto (if he exists in this fanfiction context) would be confused. They've always looked up to Naruto as a hero in Konoha. His leaving would shatter their image of him a bit. But the older, more experienced friends like Kakashi would likely be more empathetic. He has seen Naruto grow and knows that Naruto must have a strong motivation for leaving. He might even offer some silent support from afar, believing that Naruto will return when the time is right.
Some might be shocked and feel betrayed. Harry is a hero to many, so his departure could leave them feeling abandoned. Others might understand his need for a break and respect his decision.
If his friends continue to ignore him, Harry could become sad and withdrawn himself. He might start to isolate more, spending time alone in places like the Forbidden Forest or an empty classroom. He might also lose his confidence in himself, thinking that he is not worthy of their friendship. In his loneliness, he could focus more on his studies or some solitary magical pursuits, trying to distract himself from the pain of being ignored.