He might be shocked at first. Then he could feel a strong sense of protectiveness towards Harry. Maybe he immediately tries to find ways to keep Harry safe from further harm, like staying closer to him at all times.
After the initial shock and sadness, Twilight often becomes determined. She decides that she will not let the betrayal break her. In a story where she is abandoned in a dangerous realm, she uses her magic and wits to find her way back and also to expose those who betrayed her, showing that she is stronger than they thought.
They would be shocked. The wizarding society has its own set of norms and values. Such an unexpected situation would likely cause a great stir, with many people being judgmental and confused.
Bella might go into a deep depression. She was so in love with Edward, and his death could shatter her world. She could become withdrawn and isolate herself from her friends and family.
Edward would likely be in a state of panic. He loves Bella deeply, so he'd probably do everything in his power to save her. He might search for a cure or a way to reverse whatever is causing her to die.
Edward would be completely devastated. He would likely go into a blind rage and seek out whoever was responsible for Bella's death. His love for her was so intense that he would stop at nothing to avenge her.
However, some human Cullens might be a bit perturbed. If a fanfiction misrepresents them or their family values in a negative way, they could feel a bit of annoyance or disappointment. For example, if a fanfiction portrays them as being cruel for no reason.
The Twilight characters would be quite shocked. Vampires like Edward might be very suspicious of Hermione's new status as an imprinted character. Werewolves like Jacob would have a different reaction. Since imprinting is a big part of their culture, they might be more accepting but also cautious as Hermione is an outsider from a different fictional universe. They would need to figure out how to incorporate her into their pack or deal with her in some way.