Snape's initial reaction would be one of disbelief. He has his own set of prejudices and ideas, and gay romance fanfiction is far from his realm of understanding. He might snatch the fanfiction away from Harry in a fit of annoyance. But as he sees Harry crying, he might start to wonder what in this strange piece of writing has such an emotional impact on Harry. This could lead to a very awkward and tense moment between them, with Snape not knowing how to handle the situation properly.
In some fanfictions, Snape might be secretly sympathetic. He may see the scars as a reminder of all Harry has been through, which could soften his usually harsh attitude towards Harry, at least for a moment.
Harry's reaction would be complex. He has so many emotions tied up with Snape already - dislike, confusion, and a bit of grudging respect in some ways. Discovering he's his father would bring all those emotions to the surface and more. He might go through a period of denial, not wanting to believe it at first. Then, as the truth sank in, he'd have to come to terms with a new family member. He might also feel a sense of betrayal from those who knew or suspected but didn't tell him. And he'd have to figure out how to fit this new relationship into his life, especially considering his role in the wizarding world and his ongoing battles.
He would probably be very displeased. Snape has always been strict and traditional, and fanfiction about the DA, which was an unauthorized student group in his eyes, would seem like a frivolous and inappropriate thing to him.
This is a very disturbing scenario. In the context of the Harry Potter universe which is generally about magic, friendship, and good vs. evil, such a concept goes completely against the spirit of the original work. Snape would likely be extremely horrified and disgusted, given his complex relationship with Harry. He might see it as a desecration of the students and the world he knows.
In the fanfiction world, Snape might be initially shocked. He has a complex relationship with Harry. He could feel a sense of protectiveness that he rarely shows openly. He might use his skills and knowledge to find the perpetrator and seek justice for Harry.
The discovery could have a very unexpected impact on their relationship. Snape, being the complex character he is, might see Harry's vulnerability in this moment as something that humanizes him. Harry, in turn, might expect Snape to be cruel about it but if Snape isn't, it could start to break down some of the barriers between them. They might not become friends overnight, but it could be the start of a change in how they view and interact with each other. For instance, Snape might be a bit more lenient in his treatment of Harry in class after this incident, or Harry might start to think that there's more to Snape than his usual grouchy exterior.
Well, Snape is a complex character. If he discovers that Harry was raoed in a romance fanfiction, he would probably be filled with a mixture of emotions. First, there would be disgust at the very idea of such a thing happening to Harry. Then, he might feel some guilt, as if he should have been able to prevent this (even though it's in a fictional scenario). He would most likely want to confront the person responsible for writing it, and he would also be more vigilant about Harry's well - being. He might even start to see Harry in a new light, with a deeper understanding of the potential vulnerability that Harry could have.
Snape would be appalled. He has a deep connection to the world of Harry Potter, and the idea of a rape romance fanfiction is so far removed from the values of the wizarding world he knows. He might initially be in disbelief, thinking it was some sort of dark joke or a misunderstanding. But once he realizes it's real, he would likely feel a mix of anger towards the creator and sadness that such a thing exists in relation to a world he cares about.
He might be quite shocked at first. After all, Wolfstar involves his dad's best friend Sirius and his other dad figure Lupin. It's a rather strange concept for him to think about them in a romantic way as he knew them in a different context.
Cedric would be deeply troubled. In the wizarding world, they have their own code of honor and respect. Finding out about Harry's abused fanfiction would go against everything Cedric stands for. He might start to question the morality of the Muggle world that allows such things to be written. He would also feel a sense of sadness for Harry, knowing that someone out there has created a story that portrays him in such a negative and harmful light. Cedric might even consider taking some sort of action, like talking to Harry about it or trying to get the fanfiction removed if he could, though that would be a complex matter given the nature of fanfiction and the different worlds they live in.