Well, Annabeth is a strong character. When she finds out Percy was abused in the fanfiction, she likely starts by trying to comfort him. She may use her intelligence to figure out how to help him deal with the trauma. Maybe she'll also plot some sort of revenge against the abuser in her own clever way, all while being a great source of emotional support for Percy.
Perhaps there could be a plot where Annabeth discovers that the abuse has given Percy some hidden powers or a different perspective on his demigod abilities. So she encourages him to explore and master these new aspects of himself. Or, it could be that the news of Percy's abuse spreads among their friends, and they all come together to form a support system for him, which in turn strengthens their group as a whole.
In this kind of fanfiction, there could be a lot of shock and disbelief among the characters at first. His friends might feel guilty for not noticing the signs earlier. They would probably become extremely protective of Percy.
His family member, if there is one in the story who is perceptive and caring. They might have seen some small signs over time but not fully understood what was going on. Once they find out about the abuse, their natural instinct to protect their family member would kick in, and they would be one of the first to react, showing concern and a determination to make things right for Percy.
The abuser might find themselves ostracized by the whole demigod community. Word would spread about what they did, and no one would want to be associated with them. Also, they could be targeted by other protective figures in Percy's life, not just Poseidon. The gods and goddesses who have some connection to Percy or just have a sense of justice would make sure the abuser pays for their actions. They could be cursed to have a series of misfortunes befall them, like always being in the wrong place at the wrong time and facing various dangerous situations.
Well, one possibility is that Artemis would become extremely protective of Percy. She might use her powers to shield him from any further harm. She could also seek revenge on those who abused Percy. In the world of fanfiction, this could lead to some really interesting power plays and conflicts.
She would probably be furious. Artemis has a strong sense of justice and protecting the vulnerable. So, she would likely immediately start to plan how to deal with those responsible for Percy's abuse.
The abusers might be ostracized by the community. Once Triton finds out and spreads the word, other sea - creatures or characters in the story might shun them. This could lead to them being isolated and having a hard time getting help or resources.