Another way Percy could react is by giving Annabeth the silent treatment. He wouldn't talk to her for a while, which would be really difficult for Annabeth as they are usually close. Percy would just go about his business, ignoring her attempts to communicate until he feels she has truly understood how much she has angered him.
In the long run, Percy might grow from the experience. He could become more cautious in his relationships and less trusting. Or, if there's a chance for redemption in the fanfic, he might be open to forgiving Annabeth if she shows true remorse and has a valid reason for her actions. But initially, it would be a very difficult and emotional time for him.
Percy's reaction could be complex. Initially, he'd be hurt and angry. But as time passes, if Annabeth is sincere in her attempts to get him back, he might start to question his own feelings. He could be torn between his love for her and the pain of the betrayal.
He'd probably be devastated. Percy loves Annabeth deeply, so this would be like a punch in the gut for him.
Percy could also be the type to demand an explanation from Annabeth. He wouldn't just let it go without trying to understand why she did it. He's not a pushover, and his relationship with Annabeth was important to him, so he would want to know what led to this betrayal.
She'd probably be shocked at first. But then her face would light up with joy. After all, she and Percy have been through so much together.
Percy might be a bit flustered and embarrassed. He's protective of his relationship with Annabeth, so having Magnus walk in might make him feel like their privacy has been invaded. He could quickly try to shield Annabeth or just look really shocked.
After the initial shock and anger, she would start thinking rationally. Annabeth is known for her strategic mind. So she'd start observing the situation, looking for any weaknesses in the captors' setup, any tools she could use to her advantage, all while trying to keep Percy's spirits up during the torture.
He could become withdrawn. Percy is a sensitive person at heart. So if Annabeth's actions are really hurtful, like if she cheats on him in a fanfic (although it's a very sad scenario), he would likely isolate himself from her and their friends. He might go off on his own to think things through and deal with his pain privately.
At first, Percy would likely be angry. He's a fighter, and being betrayed by someone he loves would fuel his rage. He might go after Annabeth to demand an explanation. But as the reality sinks in, he could become very sad and withdrawn. He might isolate himself from his other friends as well, feeling like he can't trust anyone anymore. He would constantly be thinking about what he did wrong to make Annabeth turn on him.
He would probably be shocked. Percy has always trusted Annabeth, so being stabbed by her would be completely unexpected. He might initially be unable to believe what's happening.