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.
He could feel a great sense of betrayal. Percy and Annabeth have a deep relationship, and this act would shatter that bond. He might question everything they've been through together.
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 But as the reality sinks in, he could become very sad and withdrawn.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.
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.
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.
Well, it completely shatters their trust. Percy would be devastated and feel extremely betrayed. He would question all their past interactions and wonder if Annabeth ever really cared about him.
Percy would feel a deep sense of guilt as well. He might think that he should have been there to protect her. So, he would be determined not only to free her but also to make sure whoever is responsible pays for what they've done. He would use his powers over water to his advantage, like creating distractions or using water to weaken the enemy's defenses while he tries to reach Annabeth.
He could be a bit shy. Since he also has feelings for Annabeth but was too nervous to make the first move. So when Annabeth asks him out, he blushes and stammers a bit, like, 'Uh, yeah, Annabeth. That sounds great.' And then he would be really happy inside, thinking about all the things they could do together on their date.
It could also be due to some sort of magic or mind - control. A powerful enemy might have cast a spell on Annabeth, forcing her to turn against Percy. In this case, it's not really her own will but the result of external manipulation.
Annabeth would likely be furious at first. She has been through so much with Percy, and for him to leave her for Calypso would seem like a betrayal. But then, she might also start to reflect on their relationship and what went wrong. She could become determined to win Percy back or to move on and prove that she doesn't need him.
One possible reason could be a misunderstanding. Maybe she saw Percy doing something that she misinterpreted as a betrayal of their friendship or their cause. For example, if Percy was seen talking secretly with an enemy, Annabeth might jump to conclusions without knowing the full story.
He could also become withdrawn and depressed. Percy and Annabeth have a very strong bond, and her death would leave a huge void in his life. He might isolate himself from his friends and family, lost in his grief and memories of her.