Hermione might be initially very hurt and angry. She's a proud and strong character. She could cut Draco off completely and refuse to have any communication with him. When Draco shows regret, she may be skeptical at first. After all, he has broken her trust. But deep down, she might still have some feelings for him, so she could be in a state of internal conflict.
At first, Hermione would likely be devastated. She may cry and feel a great sense of betrayal. Given her intelligence, she might analyze the situation over and over in her head, trying to figure out what went wrong. When Draco expresses regret, she could be cold towards him. She might demand an explanation and a sincere apology. However, it would take a lot for her to forgive him, as she values loyalty highly. She might also start to focus more on herself and her own growth, not wanting to be held back by Draco's actions.
He might regret it because he finally understands that Hermione was the one who truly accepted him for who he was. In fanfiction, often there's a development where characters see beyond the surface. Draco, being from a family with certain expectations, may have found in Hermione someone who didn't care about his family's dark past or his initial snobbishness. When he cheats, he breaks that bond of acceptance, and then realizes how special it was. Additionally, he could be regretful because he sees Hermione move on and be happy without him, and he misses the relationship they had.
Hermione could become withdrawn. She might isolate herself from her friends for a while, spending her time in the library trying to make sense of it all. She might also feel a sense of betrayal, especially if they had some sort of understanding or relationship developing.
Well, Draco could be very conflicted. On one hand, he has his own pride and his Slytherin upbringing which might tell him to stay away. But if he has developed deeper feelings for Hermione in the fanfic, he might put aside their past differences. He could start by using his family's resources to find a cure or at least better treatments for her cancer. He may also become more protective of her, keeping her away from those who might be unkind about her illness.
She might be initially shocked. After all, Draco is usually seen as this tough Slytherin. But then she could feel sympathy. For instance, she might offer him a handkerchief without saying much at first.
She might be heartbroken at first. Hermione is someone who invests deeply in her relationships. She would likely retreat into her studies as a way to cope, burying herself in books to try and forget the pain.
Draco might be initially shocked. He's used to seeing Hermione as a strong adversary, so seeing her cry would be unexpected. He could just stand there, not knowing how to react at first.
Hermione may initially become very withdrawn and focus more on her studies and magical research after Draco cheats. She throws herself into her work as a way to cope with the pain. However, as Draco tries to win her back, she has to face her feelings again. In the process, she learns more about herself and her own needs in a relationship. She discovers that she can be strong enough to forgive and that love doesn't have to be perfect. This new - found understanding helps her to grow as a person and also in her relationship with Draco. She becomes more confident in her decisions regarding love and forgiveness.
Hermione might initially be furious. She is a strong - willed character, so being tied up against her will would surely make her angry. She would probably start using her wits right away, trying to figure out a way to free herself, whether it's through magic or by tricking Draco.
Hermione could also be more understanding in some fanfictions. If she senses that there's something deeper behind Draco's actions, she might try to calm the situation down. She could ask him why he did it instead of immediately retaliating. Maybe she sees a glimmer of vulnerability in his eyes and decides to approach the situation in a more empathetic way. This could then lead to a conversation that changes their relationship for the better.
She might be heartbroken. Hermione is a strong - willed character, but being cheated on would surely shatter her. She could initially be in disbelief, given her trust in Ron. Then, she might become angry and start to distance herself from Ron, focusing more on her own studies or other relationships, like her friendship with Sirius.