Sakura would be devastated. She'd immediately use all her medical knowledge to try to find a cure. She might also be angry at herself for not noticing earlier. Kakashi would be very solemn. He'd use all his experience and connections to look for a solution. Shikamaru would start formulating plans to find the cause of Naruto's condition. He'd also try to keep the other friends calm while they search for answers.
Sasuke, when he hears about Naruto's condition, would rush back. He'd feel a sense of guilt for not being around. He'd vow to do whatever it takes to save Naruto. The other Konoha 11 members would also be heartbroken. They'd start looking into old scrolls and legends for a way to reverse Naruto's condition. They'd hold on to hope and encourage Naruto to keep fighting. They'd also make sure that Naruto's last days are filled with love and memories if they can't find a cure in time.
Maybe Naruto is affected by a dark and ancient curse. This curse was placed on him by an unknown enemy. It starts to take hold gradually, making him weak and sick. He begins to have strange visions and nightmares. His body slowly deteriorates, and he loses his appetite. His friends notice his strange behavior and the physical changes, but Naruto is determined to keep fighting until they find out what's wrong with him.
Well, it might start with Naruto feeling weak and tired more often. His friends would notice his decline but he would try to hide it at first. As the story goes on, he would start losing his ability to perform jutsus as effectively.
Naruto's friends would react in a very emotional way. Hinata, being his wife, would be devastated. She would stay by his side constantly, taking care of him and their children as best she could while also hoping for a miracle. His friends from the Rookie 9 would all come together. Kiba might bring Akamaru to try and cheer Naruto up, while Shino would be more quiet but still show his support. They would all band together, using their unique skills and abilities, whether it was in combat, medicine, or just emotional support, to try and make Naruto's last days as good as possible.
One possible reason could be a powerful curse placed on him. Maybe an enemy with dark magic managed to put a slow - acting curse that is gradually sapping his life force.
Sasuke's reaction would be the most complex. He would be filled with guilt for causing Naruto's death. He would also feel a new sense of purpose, to live up to Naruto's sacrifice. Sasuke might become more withdrawn at first, dealing with the enormity of what has happened. The other ninja from Naruto's generation, like Shikamaru, would be sad but also respect Naruto's choice. They would support Sasuke in his new path, as they understand it was what Naruto wanted. And the younger generation would see Naruto as a martyr, someone who sacrificed for a greater good.
Sasuke might initially be a bit aloof but secretly be happy for them. He's always had a complex relationship with Naruto and Sakura, and deep down, he would see that they are good for each other.
His friends like Kakashi would be cautious yet supportive. Kakashi would likely give Naruto advice on how to handle the relationship and the responsibilities that come with it. In Kirigakure, Mei's comrades might initially be distrustful of Naruto, but as they see his good intentions and how he treats Mei, they would gradually accept him. And Naruto's friends from other villages he has befriended over the years would see it as a positive step towards a more unified ninja world.
Kakashi would be more of a supportive figure. He has seen Naruto and Sakura grow up, and he would likely be happy for them. He might offer some advice to Naruto on how to handle this new relationship, especially considering the challenges they might face as ninjas. For Sakura, he could give her words of encouragement to be true to her feelings.
At first, Naruto might be in shock. He has always trusted his friends, so the sudden betrayal would be a huge blow. He could be silent, unable to process what has just happened.
Sakura might be really surprised at first. She could offer to help Naruto with his girl disguise, teaching him things like how to do his hair or choose the right clothes. She might also find it a bit funny but would be supportive in the end, seeing it as part of a mission. Sasuke, on the other hand, might be a bit more aloof. He would probably just raise an eyebrow at Naruto's new look but wouldn't make a big fuss about it as long as it was for a good reason.