Percy would probably be extremely hurt. He trusts Annabeth deeply, so this betrayal would shatter that trust. He might feel angry at first, wanting to confront both Annabeth and Luke.
Percy might become very withdrawn. After the initial shock and anger, he could isolate himself, feeling that he can't trust anyone anymore. His confidence would be severely shaken, not just in Annabeth, but in his own judgment of people. This could lead to him making some rash decisions in the fanfiction, either trying to get revenge or just trying to run away from the whole situation.
He could also be in denial. Percy might not be able to believe that Annabeth would actually betray him. He would search for reasons, thinking that there must be some sort of mistake or that Annabeth was being forced or tricked. He may try to win her back, thinking that their relationship was strong enough to overcome this setback.
Luke might become more enraged. Since Percy has thwarted his plan to do something to Annabeth, he could seek revenge. He might start plotting more devious schemes against Percy and Annabeth, or even their friends.
Well, it could be that in the fanfiction, the writer might have developed Annabeth's character in a way where she becomes disillusioned with Percy. Maybe she sees Luke as having something that Percy lacks, like a different kind of power or charm. Or perhaps there are some misunderstandings between Percy and Annabeth that Luke takes advantage of, pushing Annabeth towards the betrayal.
She might be disgusted. Gabe is such a horrible person in Percy's life. So Annabeth could show immediate anger towards Gabe just from Percy's description.
She could be extremely worried. In fanfiction, Annabeth might constantly fret over Percy's safety as Styx is a very dangerous place. She may try to find ways to reach him or send help.
She might be a bit shocked at first. But then, being Sally, she'd probably quickly regain her composure and give them a knowing smile. After all, she's seen Percy grow up and she would be happy for him to have such a close relationship with Annabeth.
Percy could be really angry. He might lash out at Annabeth and those involved. He has a fiery temper sometimes, and this would be a huge betrayal.
One possible reason could be that Kronos has somehow tricked her. Maybe he promised her something she desperately wanted, like power to protect her loved ones in a way she thought Percy couldn't. Another reason might be that she was brainwashed. Kronos could have used some dark magic or manipulation to make her believe that Percy was on the wrong side.
Well, one reason could be that in the fanfiction, the writer might want to explore a different dynamic. Maybe Annabeth gets lured by Luke's charm that has been newly developed in the fanfic. Luke could be presented as more understanding or having a different kind of allure that attracts Annabeth away from Percy.
It could be for the greater good as she perceives it. Annabeth might think that by betraying Percy, she can save more people or prevent a greater disaster. However, her perception might be wrong, and she realizes it too late after the betrayal has already occurred.
Then, she would probably be angry. Annabeth is a strong - willed character. She would feel betrayed and used, and her anger would fuel her to find out the truth. She might start her own investigation to figure out what's really going on with Percy, whether he's been influenced by someone or if there's another reason for his actions.