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.
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.
In some fanfictions, Artemis might react with intense anger. She is a protector, especially of the innocent. So she could immediately seek justice for Percy, using her power and influence among the gods to find the perpetrator.
Artemis would probably be furious. As a goddess associated with protection, especially of women and girls, she would see this as a great injustice. She might immediately start planning revenge on the one who did this to Percy.
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.
One consequence could be that the perpetrator is severely punished. Artemis would not tolerate such an act, and she would use her power to make sure justice is served.
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.