Peter might fight back if he feels threatened. He's a hero and won't just let his team harm him without a good reason. However, he'll likely try to avoid causing serious harm to his friends. He could use his agility and intelligence to outmaneuver them in a fight, all while trying to get them to see the truth.
Peter will probably be very confused at first. He's used to being part of the team, and suddenly being hunted by them is a huge shock. He might try to talk to them to clear up any misunderstandings.
One possible reason could be a misunderstanding. Maybe Peter has been acting secretively about his Spider - Man activities, and his team misinterprets it as him being up to something bad. For example, if he has to disappear suddenly to fight crime, his team might think he's ditching them for other, less noble reasons.
Peter would likely be defiant. He has a strong moral compass and wouldn't easily give in to Osborn's demands. He'd probably try to find a way to escape right away, using his spider - abilities like climbing walls and his super strength to break free from any restraints.
Peter would probably be scared at first. He's just a kid, after all, and being kidnapped by a powerful being like Thanos is terrifying. But he would also be defiant. He has a strong sense of responsibility, so he might try to find a way to escape on his own.
Peter might be initially suspicious. After all, in his world, strange things often mean trouble. But then he could also be curious to find out more about this doppelganger. He might try to test the doppelganger's abilities, like seeing if they have the same spider - sense. If the doppelganger seems friendly, Peter could start to form a bond with them, sharing stories about their lives and learning from each other.
His friends would be very worried. For example, Ned would probably be constantly by his side, trying to find ways to help him remember. He might show Peter old photos or talk about their shared memories.
Peter's scientific mind would be an asset. He could work with the team's tech - savvy members. His superhuman strength and agility would also add to the team's combat capabilities. In terms of the team dynamic, he'd be like the little brother that everyone looks out for.
First, if the fanfiction is really good, you can recommend it to your friends who are also fans of Peter Parker. You can react by being critical too. If there are plot holes or if the character of Peter seems out of place, you can write a constructive review. Another way to react is by creating fanart inspired by the fanfiction. This shows your appreciation for the story in a creative way.
Some of his minor enemies might see this as an opportunity to strike. They could think that a sick Peter Parker is an easy target. For example, Flash Thompson might try to bully him more than usual, not knowing that even sick, Peter has his powers.
In some fanfictions, the school might be in awe. His classmates could be jealous or inspired. For example, Flash Thompson might feel a mix of envy and grudging respect.
The media would probably go wild. There would be headlines everywhere. Some media outlets might portray him as a savior, while others might try to dig up dirt on him or question his motives for hiding his identity for so long.