Well, it's hard to say exactly. Maybe Mr. Dursley would be outraged at how his family is being fictionalized in these fan - created stories. He's so proper and hates anything out of the ordinary. Mrs. Dursley might be more interested in how people are writing about her cooking or how she runs the household in these fanfic versions. Dudley, on the other hand, might just be confused as to why anyone would write stories about his family at all.
In a fanfic, the Dursleys might face charges for their mistreatment of Harry. They could be put on trial by the wizarding world for things like emotional abuse and neglect. For example, they made Harry live in a cupboard under the stairs and treated him like a servant. They might be found guilty and face various punishments, such as having their magic - related memories modified or being exiled from the wizard - protected areas near their home.
They would probably be horrified. The Dursleys detested all things related to magic in the original Harry Potter story. Fanfiction might introduce new and strange magical concepts that would go against their Muggle - centric and magic - hating views.
It could go either way. If the fanfic shows Harry in a really positive light, like a super - powerful wizard who is also kind - hearted despite their treatment of him, they might feel a bit guilty. But if the fanfic portrays Harry in a negative way, they might feel justified in their own cruel behavior towards him.
If the Dursleys watched Harry Potter fanfiction, it would be a complete shock to their system. They have spent so long suppressing Harry's magic and trying to keep their world as normal as possible. Fanfiction often explores different aspects of the wizarding world, some of which might be quite extreme or different from what they think they know. They might initially be very confused, and then probably angry that such things exist. They could also try to stop Harry from having any access to it, thinking it would further fuel his 'unnatural' interest in magic.
In this type of fanfiction, one possible scenario could be that Harry's already difficult life at the Dursleys takes a fatal turn. Maybe the neglect and abuse he endures finally become too much for his young body to bear. For example, he could be severely underfed and fall ill without proper medical attention, leading to his death. It would also have a huge impact on the wizarding world. Since Harry was seen as the 'Chosen One', his death might cause chaos and despair among those who were hoping he would defeat Voldemort.
They are often portrayed as the villains. Their years of treating Harry badly are brought to light. They might look scared and out of place in the wizarding courtroom, with all its strange sights and magic.
In this fanfic, the Dursleys may be depicted as being in over their heads. Vernon Dursley, for example, might be shown as constantly angry and trying to assert his authority in a situation where he has none. Petunia Dursley could be portrayed as being torn between her dislike for magic and her sense of family obligation towards Harry. Dudley might be shown as being bullied or affected in the Muggle world because of Harry's presence in their family.
Mrs. Dursley was Mrs. Potter's sister, but the Dursleys treated Harry Potter badly. They never treated him as their own son, almost abusing him. They hid Harry's magical identity, which made Harry confused about his birth and the magical world in the first book. This kind of bad behavior was mainly psychological damage. Harry Potter and the Sorcerer's Stone was not enough. Everyone, please click to read the novel!
Maybe Luna's unique personality would shock the Dursleys. She might say some strange but profound things that leave the Dursleys completely bewildered.
In the 'Harry's kids meet the Dursleys' fanfic, the kids might be curious about their father's childhood and the Dursleys' role in it. They could confront the Dursleys about how they treated Harry. Maybe the Dursleys would be afraid of the kids because they are the offspring of a powerful wizard. It could also be a chance for the Dursleys to redeem themselves in a way, by showing some kindness to Harry's children that they never showed to Harry.