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 She could become withdrawn and start to question her self - worth.
Sakura might also get angry. She could feel that Naruto is being unfair or cowardly. She might try to find Naruto and confront him about why he left her alone. In the process, she could end up growing stronger both physically and emotionally. For example, she might train harder to prove to Naruto that she doesn't need him around. And also, she could start to re - evaluate her own feelings and relationships in general.
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.
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.
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.
Tifa might be initially heartbroken. She has a deep connection with Cloud, and his sudden departure would surely shatter her. She could spend days in a state of disbelief and sadness, constantly replaying their memories in her head.
She might be surprised at first. After all, Naruto isn't known for his singing usually. But then she could be touched if she realizes the song is for her and about their relationship, whether it's friendship or something more.
Sakura might react by being more assertive around Naruto. Instead of being shy or holding back, she could be more forward in showing her friendship or even her deeper feelings. She may also try to intimidate the fangirls in a way, like using her strength as a ninja to show that she is not one to be trifled with.
Sakura would likely be devastated. Naruto has been her friend and teammate for a long time. His death would leave a huge void in her life, and she'd probably go through a long period of mourning.
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.