Percy might initially be confused. He would probably wonder why Annabeth, who was once his friend or ally, is treating him this way. So he could try to talk to her to find out what's wrong.
Percy could be furious. He has always been loyal to Annabeth, and this betrayal would be a huge blow. He might immediately cut off all contact with her and go off on his own for a while to cool down.
One reason could be a misinterpretation of their characters. Maybe in the fanfiction, the writer wants to create a more complex or different dynamic between them. Another reason might be for the sake of drama. By having Percy bullied by Annabeth, it can create conflict and drive the plot forward in a new and unexpected way.
Annabeth could also be angry, not at Percy but at the situation or the forces that led to his death. She might vow revenge and start on a journey to find out who or what was responsible for Percy's sacrifice and make them pay. This could lead her into a whole new set of adventures and challenges as she tries to come to terms with her loss.
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.
She might be heartbroken at first. She could try to jog Percy's memory by showing him things that were important to them.
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.
Percy could initially be heartbroken. He's a very loyal person, and to be betrayed by those he considered family would be devastating. In Tartarus, he would have to rely on his instincts and his powers. He might also experience a lot of self - doubt, wondering if he really was as bad as they made him out to be. As time passes, he would start to plan his revenge against those who wronged him while also looking for an escape route.
It could be that Annabeth has a misunderstanding about Percy. She might think he's a threat to her position or status at the camp, so she bullies him to try and keep him in check.
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 But as the reality sinks in, he could become very sad and withdrawn.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.