They might be shocked. Percy is not an ordinary guy, and his abilities would be quite a surprise to them.
In the fanfiction, their reaction could also depend on the setting. If they meet Percy in a dangerous situation where his powers are needed to save them, they would probably be very grateful. But if they meet him in a more normal, social setting, they might be more cautious or curious at first. Their reactions would add a lot of drama and interesting plot twists to the fanfiction story.
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 terrified. Poseidon is a god and his power is overwhelming. They could start trembling or even faint at the sight of him.
Percy would probably be curious. He's always up for new adventures and meeting new people. He might immediately ask Harry about his magic and how it works. Annabeth would be more cautious. She'd be observing Harry, trying to figure out if he could be trusted. But once she realizes he's on the side of good, she'd start asking intelligent questions about the wizarding world, like how their education system compares to Camp Half - Blood's training.
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.
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.
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.
They would probably be shocked. Demigods have extraordinary abilities, like being able to control water or run super fast. Percy's mortal friends, who are used to a normal world, would be taken aback by such powers.