Chiron would be concerned. He knows how important their relationship has been for the stability of the camp and for their own growth as demigods. He would likely try to mediate and help them work through their issues, if possible. If not, he would worry about how it will impact their future quests and the overall well - being of the camp and its inhabitants.
The campers at Camp Half - Blood would be shocked. Some might take sides, especially those who are closer to either Percy or Annabeth. The Aphrodite cabin might be all up in the drama, trying to play matchmaker again to fix things. The Ares cabin might see it as a sign of weakness in Percy and try to pick on him a bit more. Overall, it would create a lot of gossip and tension around the camp.
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.
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 But as the reality sinks in, he could become very sad and withdrawn.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.
He could also become angry. Percy has a bit of a temper, and he might lash out at Annabeth, demanding an explanation. He might feel betrayed, especially if he doesn't understand the real reasons behind the break - up. In his anger, he could distance himself from their mutual friends and go on solo quests to clear his head.
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.
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 might be initially shocked and a bit flustered. She's a composed person, but this unexpected act from Percy would catch her off - guard. Percy, on the other hand, might be a bit proud of his 'mark' on Annabeth at first. Their friends like Grover could be really surprised and might tease them about it, while some of the more serious demigods might be a bit disapproving, thinking it's not appropriate behavior for heroes.
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.