They would all be affected in different ways. The more emotional ones like Ino would be crying and trying to comfort Naruto. Lee would be inspired by Naruto's pain to train even harder so he can be strong enough to protect his friends in the future. Neji, if he were still alive in this fanfic, would be using his Gentle Fist to defend Naruto while also looking for an opening to counter - attack the enemy.
His friends would most likely rally around him. Kiba might use his dog Akamaru to scout the area and make sure there are no other immediate threats. Hinata would be really worried and would try to use her Byakugan to see if there are any hidden enemies or if Naruto has any internal injuries. Choji would offer some of his food to Naruto to give him strength, as his clan's food can have special effects on a ninja's body.
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.
I think Naruto's friends would be quite shocked. For the more traditional ninja like Neji (if he were still alive in this fanfic), he would see the gun as an abomination to the ninja way. However, Naruto's closest friends like Kiba would be more open - minded. Kiba might see it as a cool new weapon and want to see what it can do. But overall, they would all be concerned about how Naruto got the gun and what it means for their world of ninjas.
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 react in a very concerned and protective way. Hermione would use her intelligence to find the best solutions for his recovery. She might also try to figure out what went wrong on the mission to prevent it from happening again. Ron would be there for Harry no matter what, showing his loyalty. Ginny, being Harry's love interest in many fanfictions, would be extremely emotional. She would be a strong presence by his side, making sure he has everything he needs. The rest of his friends would also come together, forming a support network around Harry, sharing memories and positive thoughts to help him through the tough time of recovery.
His other friends like Shikamaru would think of the most practical solutions. He might calculate the risks of continuing the mission with a sick Naruto and come up with a plan. Maybe he'd suggest splitting the group, with some staying to take care of Naruto and others proceeding with the mission - related tasks that can't be postponed.
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.
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.
Maybe the enemy would see Naruto's injury as an opportunity to finish him off. But Naruto has a habit of surprising his foes. Even in his hurt state, he could pull off some unexpected jutsu. His friends could also use this situation to set up a trap for the enemy. For example, they could make it seem like Naruto is more injured than he actually is, luring the enemy into a false sense of security before attacking.
Sakura, being a medic - ninja, would likely be very concerned and immediately start trying to diagnose and treat Naruto. She might be constantly by his side, using her medical skills to ease his symptoms.
Her friends usually rush to her aid immediately. Percy, for instance, would be very protective and might even get angry at whatever or whoever caused her harm. Grover would use his nature - related skills to try and find herbs or something to heal her. The rest of the group would also form a protective circle around her, making sure no further harm comes to her while they figure out how to treat her wounds.