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.
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 She could become withdrawn and isolate herself from her friends and family.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.
There are other fans who are interested in exploring different storylines. They see it as a creative way to expand the Twilight universe. They might enjoy seeing how the characters would react in such a difficult situation, even if it's not in line with the original story.
In Twilight fanfiction, when a major's mate is in danger, they would experience a strong emotional turmoil. This could translate into a very focused and fierce reaction. They might use their influence or status within the Twilight world to gather resources or help. Their reaction would be centered around ensuring the safety of their mate at all costs, and they would not rest until the danger has passed.
She might be devastated. Bella was deeply in love with Edward, so his leaving would be a huge blow. She could cry for days and feel like her world has ended.