Nico might be initially shocked. He could feel a deep sense of anger towards whoever was responsible for Percy's abuse. Then he would probably become very protective of Percy, vowing to keep him safe from any further harm.
One possible plot development could be Nico seeking justice for Percy. He might start to investigate the abuse on his own and gather evidence against the abuser.
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.
One possible fanfiction could be 'The Unseen Pain'. In this story, Percy might be crying due to the overwhelming pressure of his demigod duties. Nico stumbles upon him in a secluded corner of Camp Half - Blood. Nico, initially hesitant, decides to approach Percy and offer his silent support.
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.
I'm not sure where you can specifically find a fanfiction with that exact plot. You could try popular fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. Just search for relevant tags like 'Percy', 'Nico', and 'Will' to see if such a story exists.
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.