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 Avengers would initially be in shock when they find out Peter is sick. Clint Barton would be the one to keep a calm head and start organizing things. Wanda Maximoff would try to use her powers to ease Peter's pain if possible. Vision would use his vast knowledge to try and find a solution. They would all be very concerned and do everything they can to make Peter better, from getting the best medical help to providing a safe and comfortable environment for him to recover.
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.
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 relationships can also be developed. His friends could come to visit him when he's sick. He might be more open with them about his insecurities as Spider - Man. And his enemies might take advantage of his weakened state, which forces him to grow and find new ways to defend himself even when he's not at his best.
Well, in fanfiction, his classmates' reactions can vary widely. There are those who are his friends and they would be supportive. They might cover for him when he has to run off to be Spider - Man. On the other hand, there could be classmates who are envious. They might spread rumors about him or try to expose his secrets just out of spite or envy. And then there are the classmates who are just indifferent. They don't really care about Peter's secret life and just see him as an ordinary classmate most of the time.
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.
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.
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.