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.
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 But as the reality sinks in, he could become very sad and withdrawn.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.
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.
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.
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.
Well, perhaps Percy plans this elaborate scheme to ask Annabeth out. He might enlist the help of their friends, like Grover. Grover could set up a picnic in a beautiful meadow. Percy then shows up with a bouquet of Annabeth's favorite flowers and asks her out. She would be touched by the effort he put in and say yes. Then they start planning all the things they want to do together on their first official date.
Well, some of their friends might be really excited for them. Hazel would see it as a positive development, thinking that they are a great couple. Frank might be a bit shy about the whole thing, but still give them his祝福 (blessing). However, there could also be some friends who are a bit concerned. Thalia, for example, might worry that their relationship could get in the way of their demigod duties. She would be afraid that they would be too focused on each other and not on the important tasks at hand. But overall, most of their friends would probably adjust to the new relationship fairly quickly.
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.
Annabeth would probably be very hurt at first. She has a strong bond with Percy, and his leaving for someone else would be a huge blow to her self - esteem and her heart.