The younger demigods at Camp Half - Blood would look up to them even more. They would see Percy and Annabeth as the ultimate couple. Their friends from different cabins would all come together to give them gifts. Some might give them weapons or magical items they've crafted, while others would offer to help with the wedding preparations, even though it's just an engagement at this point. Their friends' reactions would range from pure excitement to a bit of friendly envy.
Their friends at Camp Half - Blood would be overjoyed. Grover would probably be dancing around and playing his reed pipes in celebration. The other demigods would see it as a sign of hope, that love can thrive even in their dangerous world. They would start planning a huge party with lots of food and games.
Grover would be super excited. He'd probably start bleating happily and immediately start thinking about all the great food he could eat at the wedding. Jason would be very supportive and offer his help in any way he could, like with security for the big day. Piper would be thrilled and would probably start planning Annabeth's bachelorette party right away. Leo might crack some jokes about Percy finally tying the knot but deep down he'd be really happy for them.
Grover would probably be really excited. He might bring all kinds of nature - related gifts for the baby, like a special plant that has protective powers. He would also be very protective of the baby, using his nature senses to detect any danger.
Jason and Piper would be very supportive. They'd offer their help, like babysitting or providing resources. Piper might use her charm - speak ability to soothe the baby when it's fussy, and Jason could use his leadership skills to help Percy and Annabeth make important decisions about the baby's future.
Some of Annabeth's friends could react with laughter and jokes, while others might be a bit flustered and not know how to act. It really depends on the personalities of the friends in the fanfiction.
They might be shocked. Percy is not an ordinary guy, and his abilities would be quite a surprise to them.
His friends could also be divided. There might be those who immediately embrace Annabeth because they see her as an asset. For instance, if there are any students interested in architecture or strategy (which Annabeth is good at), they'd be thrilled. However, there could be some who are more territorial and feel like she's invading their space, which would create some interesting relationship dynamics in the fanfiction.
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.