Well, Naruto is a really perceptive guy deep down. If Sakura pretends to love him in fanfiction, at first he might be super excited. He'd probably start thinking about all the things they could do together, like going on dates to Ichiraku's. However, Naruto also values honesty a great deal. Once he starts to suspect that Sakura's love isn't real, he would likely confront her. He might say something like 'Sakura - chan, I feel like there's something not right. You can tell me the truth.' And depending on how Sakura responds, Naruto could either be really sad and withdraw a bit, or he could try to figure out why she's pretending and help her if there's a deeper issue.
At first, Naruto might be extremely shocked. He trusts Sakura as a teammate and friend, so being kidnapped by her would be completely unexpected. He would likely try to talk to her, asking her why she's doing this.
Naruto might be very forgiving right away. He has always had a big heart and cared deeply for Sakura, so he could just accept the apology with a big grin and say it's all in the past.
Naruto could also feel a deep sense of hurt. After all, he has been through a lot with Sakura. In the face of her betrayal, he might start to question his own judgment of people. However, being Naruto, he would not give up easily. He might try to find a way to redeem Sakura or at least make her see the error of her ways. He would draw on his own experiences of being ostracized in the past and use that to try to understand her actions and find a solution.
She might be heartbroken. Sakura could initially be in shock and then fall into a deep sadness. She may blame herself for Naruto's leaving, thinking that she could have done something differently.
Naruto might initially be shocked and heartbroken. He may try to hide his pain and put on a brave face, as he has always done in the face of adversity. He could also become withdrawn for a while, distancing himself from Sakura as a form of self - protection.
Naruto might be shocked at first. He has always seen Sakura as a friend, so being beaten up by her would be unexpected. He could try to talk to her and figure out what's wrong.
Well, Naruto might be furious. He's always been protective of Sakura. He'd probably drop everything he's doing and go after Sasuke to rescue Sakura. Naruto has a strong sense of justice and loyalty, especially towards his friends. And Sakura is one of his closest friends, so he won't stand by and let her be in danger.
He may try to talk to Sakura and understand her reasons. Naruto is a character who believes in the goodness of people. So, instead of getting angry immediately, he would likely want to resolve the issue. He could also become determined to prove himself to Sakura, that whatever she thinks he has done wrong, he is actually not that person.
However, in other cases, Naruto could be hesitant. He might have grown so used to their friendship that the idea of a romantic relationship with Sakura is a bit overwhelming. He could take a step back and say something like 'Sakura, this is really unexpected. I need some time to think about this.' This would then lead to a more complex exploration of their relationship in the fanfiction.
Naruto's reaction could be complex. Since he has his own feelings for Sakura, this situation could stir up a lot inside him. He might initially be a bit hesitant in how to approach her because he doesn't want to overstep any boundaries. But in the end, his natural instinct to help a friend would take over. He would listen attentively to Sakura, his eyes full of determination to make her feel better, and use his own unique way of thinking to give her advice or just be a shoulder to cry on.