Sakura might be completely shocked at first. She might freeze for a moment, not believing what's happening. Then she could start crying tears of joy and say yes immediately.
Sakura might initially be confused. She would probably wonder why Naruto is acting this way all of a sudden. For example, she might try to get his attention by calling his name a few times, but when he doesn't respond, she could be left feeling a bit bewildered.
She could be heartbroken. Sakura has always loved Sasuke deeply, so his leaving for Naruto would be a huge blow to her. She might initially be in denial, not believing that Sasuke would choose Naruto over her.
Naruto might also try to remember Sakura in a positive way. He would cherish the memories they had together. He could start initiatives in the village in her name, like promoting medical ninja training more, since Sakura was a great medical ninja. He would slowly turn his grief into a force for good in the village.
He could become very determined to find out why Sakura left and try to win her back. Naruto is known for his perseverance, and he wouldn't give up easily on his family. He might also start to question his own actions and wonder if he could have done something differently to prevent this.
She could be confused at first. She might not understand why Naruto is suddenly ignoring her, since they were friends before. So she might try to approach him to ask what's wrong.
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.
Some fans are horrified. They love the characters as they are in the original series and any deviation that involves such a violent act against a main character like Sakura is unacceptable to them.
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.
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.