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.
It could also be that Naruto is forced to leave Sakura due to some external threat or danger. For instance, if there is an enemy targeting Naruto and he fears that being around Sakura will put her in harm's way, he would choose to leave to keep her safe. This is in line with Naruto's selfless nature as seen throughout the series.
Naruto might initially be in denial. He's so used to having Sakura around and believing that they have a future together. So when she leaves for Sasuke, he might think it's just a temporary thing or that she's been influenced by something or someone. But as time passes and he realizes it's real, he would be deeply hurt. He might also start to question his own self - worth, thinking that he wasn't good enough for Sakura. And he might even try to win her back at first, but then slowly come to accept the situation and focus on his other responsibilities like being a ninja and protecting the village.
She might be initially shocked and hurt. Sakura has always had Naruto as a constant in her life, so his sudden absence would be difficult to accept. She could become withdrawn and start to question her self - worth.
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 might be shocked at first. Sakura has always known Naruto as someone who is very attached to Konoha. If his leaving is because of her, she could feel guilty. She might try to stop him, asking him why he's doing this and telling him that he doesn't have to go.
Another reaction could be that Sakura tries to follow Sasuke. She believes that she can change his mind or that she can be of help to him. She might leave her own responsibilities behind for a while to chase after him, thinking that their bond is strong enough to overcome whatever is making Sasuke leave.
One possibility is that Naruto realizes Sakura has feelings for Sasuke that he can't change, so he decides to focus on his own ninja journey alone and leaves her to figure out her relationship with Sasuke. He might feel that his presence is only causing confusion or getting in the way of her true feelings.
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.
One interesting Sakura leaves Naruto fanfiction could be about Sakura using her medical skills and knowledge of plants, like sakura leaves, in a unique way during a mission. For example, she might use them to create a special healing poultice or a tracking device. Another could be a story where Sakura and Naruto have a moment of peace under a sakura tree filled with falling leaves, and they share their dreams and fears.