They would likely be in shock at first. Allison might feel a deep sense of loss, as Stiles was always there with his quick thinking and humor. Derek, who had grown to respect Stiles over time, would feel a mixture of anger and sadness. They would all come together, though, to try and figure out what happened and if there was any way to bring him back, or at least get justice for his death.
The friends would experience a range of emotions. Some would be in denial, refusing to believe that Stiles is really gone. Others would immediately start looking for answers. They might split up to cover more ground in their investigation, but also find that they need to be together to support each other through the difficult time. Their relationships with each other might also change, as they are all forced to face the reality of Stiles' death in their own ways.
His friends would be devastated. Scott, for example, might go into a state of extreme anger and blame himself for not being able to protect Stiles. Lydia could be inconsolable, using her banshee powers to try and find some connection to Stiles in the afterlife.
Sonic would be devastated. He and Tails are like brothers. Sonic would likely go into a rage and vow to avenge Tails' death. He would immediately start hunting down whoever or whatever was responsible. Amy would be heartbroken as well. She would probably be the one to try to comfort Sonic while also being deeply sad herself. Knuckles would be angry and ready to fight alongside Sonic to seek justice for Tails.
His friends would be extremely worried. They might immediately drop whatever they are doing to rush to his side. For instance, Thomas would be right there, trying to assess the situation and figure out how to help. He would be filled with a sense of urgency and determination to get Minho the care he needs.
His friends would most likely be very worried. They would gather around him, trying to figure out what was wrong. Percy might use his water powers to cool Leo down if he had a fever. Annabeth would be researching in the library, looking for any information on demigod illnesses. Hazel could use her powers to sense if there was any dark magic involved in Leo's sickness.
Hermione would usually be very worried and immediately start looking for ways to heal him. She might use her knowledge of magic to find the right spells or potions to treat his injuries.
In most 'Naruto gets sick' fanfics, his friends are very concerned. For example, Sakura will immediately start using her medical skills to diagnose and treat him. Sasuke, although he may not show it outwardly as much, will stay by Naruto's side and keep a watchful eye. Shikamaru might come up with plans to make Naruto more comfortable during his illness. And Hinata, she would be really worried and would probably bring Naruto some home - cooked food to help him regain his strength.
Yamamoto would be more level - headed. He would first try to find out the details of Tsuna's arrest. He might talk to the police or other authorities to see if there's been a misunderstanding. He'd also be a source of support for Tsuna's family during this tough time.
Well, Betty will be super worried. She's always had a soft spot for Archie. She might skip her own activities to stay by his side, constantly checking his temperature and making sure he's comfortable. Jughead, on the other hand, will try to keep things light. He'll tell Archie stories about the latest strange things he's seen in Riverdale to distract him from his illness. Veronica will be more of the practical one. She'll call in doctors or specialists, using her family's influence to ensure Archie gets the best care possible. All in all, his friends will rally around him.
Sasuke would be extremely enraged. He has a deep bond with Naruto, and Naruto's death would likely send him on a quest for revenge against Konoha. He might see it as a betrayal of everything Naruto stood for and fought for.
I think his friends' reactions would be mixed. Kakashi would be that calm and collected presence, giving Naruto some words of wisdom about this new chapter in his life. Hinata might be a bit sad at first because she had feelings for Naruto, but she would also be happy for him. The rest of the Konoha 11 would be excited. They would see it as an opportunity for Naruto to bring about new changes not just in Konoha but in the whole ninja world. They would all gather around to support Naruto and make sure the marriage goes smoothly.
At first, Naruto would be in disbelief. He has always trusted his friends wholeheartedly, so this would be a huge blow. He might then feel a deep sense of anger and sadness. However, Naruto being Naruto, he would likely also start to look for ways to prove his innocence or the misunderstanding. He might go on a journey of self - discovery to figure out what went wrong and how he can win back his friends' trust, all while dealing with the pain of the betrayal.