Sakura might tease Naruto about it a little. She'd probably think it's a bit strange that he's interested in something so different from their own world. But she'd also be curious to know what it's all about.
Shikamaru would probably just shrug it off. He's used to Naruto's odd interests. He might say something like 'Well, that's Naruto for you. Always finding something new to get into.' But deep down, he might also be a bit intrigued by the idea of RWBY fanfiction and might even ask Naruto to tell him more about it later.
Sasuke would likely act like he doesn't care much. But secretly, he might be a bit intrigued as to why Naruto is into such things. He might think it's a waste of time or he could see it as another way Naruto is different from him.
Sakura might be a bit surprised at first. She would probably tease Naruto a little about it, but then she'd also be curious to see what's in those fanfics. She might even start reading some herself to see how the fans portray her relationship with Naruto.
Sasuke would probably be dismissive at first. He'd think it was a waste of time and Naruto should focus more on their ninja goals. But secretly, he might be a bit curious himself. Sakura would likely be surprised and might tease Naruto a bit about it, saying he should be more interested in medical ninjutsu or something more relevant to their village. Shikamaru, on the other hand, would just sigh and say it's too troublesome for him to care, but deep down he'd understand Naruto's need for something different.
His friends might be really shocked at first. They know Naruto as a ninja, not someone who sings fanfiction. Some might find it hilarious and start teasing him in a friendly way. For example, Shikamaru might say something like 'Only you, Naruto, would come up with something so strange.'
If their friends found out, Kiba might tease Naruto a little bit, but in a friendly way. He respects Hinata and Naruto, so he'd be okay with it. Ino would be all excited and would start asking Hinata for details. She's always interested in romantic stuff. Lee would be super positive and give Naruto a big thumbs - up, saying something like 'Youthful love is wonderful!'. And Neji, if he were still alive, would be very protective of Hinata at first, but then see how happy Naruto makes her and be supportive.
Sakura would probably tease Naruto a little bit. She might say something like 'Naruto, you're such a dork for reading that fanfiction.' But then she would also be interested in what it's about and might ask Naruto to tell her some of the cool parts.
Sakura would be furious. She's always been protective of Naruto in the original Naruto series. In the RWBY fanfiction, she would probably start her own investigation to find out who was behind the betrayal. She might even challenge the RWBY characters if she thought they were involved. Kakashi, on the other hand, would be more calm and collected. He would try to analyze the situation rationally and figure out the best way to help Naruto.
He might be surprised. He's used to his own life and adventures, and seeing it twisted and turned in fanfiction could be unexpected.
Sasuke would be furious. He already has a strong sense of justice and loyalty to Naruto. He'd likely immediately start looking for Naruto, using his Sharingan to track any possible leads. He might even go against the village rules if he thought the village was involved in Naruto's kidnapping.
His friends would be shocked at first. Sasuke might feel a sense of responsibility to find Naruto and bring him back. Sakura would probably feel guilty, especially if she was somehow involved in the situation that led to Naruto being cheated on.