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.
She may feel angry at first. Sakura is a strong - willed character. She might think that Naruto has made this decision without considering her feelings. In the fanfic, she could go on a rant about how he should have talked to her before making such a big decision.
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 has known Naruto for a long time and his sudden coldness would be unexpected. She may try to reach out to him to find out what's wrong.
At first, Naruto would likely be extremely shocked. He has always considered Sakura a friend and teammate, so this would be a huge blow. He might question his own judgment in trusting her.
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.
Sakura may react with a sense of urgency. She would drop whatever she was doing and rush to Naruto's side. She'd be worried that something truly terrible has happened. If Naruto is hesitant to talk, Sakura might just sit quietly with him, waiting for him to open up. This would show her patience and understanding, and also how much she values Naruto as a friend.
Sakura would be devastated. She would probably blame herself for Naruto's death since he died protecting her. She might go into a state of deep depression at first, withdrawing from everyone.