Well, Percy has seen a lot in his adventures. At first, he might be a bit taken aback because fanfiction is a whole new world compared to his real experiences. However, as he realizes that it's just fans exploring different scenarios with the characters, he could become rather indifferent. He might even start to see the value in it, as it shows how much the fans care about the overall story and its characters. He might think it's a little strange that people are so interested in writing about Nico and Will's relationship, but he'd respect the creativity of the fans.
I don't think it's a very common or popular theme. The Percy Jackson fandom usually focuses more on positive relationships and adventures among the characters. Choking is a rather dark and extreme concept that doesn't really fit the general tone of the fandom most of the time.
Well, in some versions, Percy might be initially shocked. He could be like 'Whoa, I had no idea!'. Then he might start to see Nico in a new light, with more understanding and compassion.
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.
Another reaction could be that Percy is worried about Sally getting into trouble because of him. He might try to convince her to stay out of it, fearing that the abuser or those associated with the abuser could harm Sally. But at the same time, deep down, he wants her support and protection.
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.