It could bring them closer as Bill becomes more protective and supportive of Harry.
It would likely worsen their relationship at first. Aunt Marge is very set in her ways and has a narrow view of the world. Discovering Harry is a wizard would be a huge shock to her system, and she would probably be very unkind to Harry.
At first, Harry might be shocked and feel a bit of pity for Malfoy. This could lead to a temporary truce between them. Instead of their usual animosity, Harry might try to help Malfoy find a way to break the curse. Their relationship could shift from being enemies to reluctant allies.
Draco Malfoy himself would be very surprised if he found out. He might initially think it's some kind of joke or a ploy to embarrass him. But as he realizes Harry is sincere, he could be conflicted. On one hand, he's been taught to dislike Harry, but on the other, this new revelation could make him see Harry in a different light too. As for the other Hogwarts students, some might be amused, while others who are more loyal to the traditional Gryffindor - Slytherin divide might be appalled.
At first, it might be a bit awkward as they come from different generations in the wizarding world. But as they spend more time together, they start to understand each other's strengths and weaknesses. Harry admires Bill's knowledge of magic and his courage, while Bill is drawn to Harry's selfless nature.
The relationship may become more complicated. Severus might struggle with his own emotions. He might feel an urge to protect Harry from this fictional abuse, but at the same time, he has to deal with his long - standing animosity towards Harry. So, it could lead to a very strange and tense dynamic between them, with Severus trying to balance his new - found protectiveness and his old grudges.
It could strengthen their relationship. Charlie's newfound knowledge might make him more committed to Harry, leading to a deeper bond of trust.
Bill might be in shock at first. He couldn't believe such a terrible thing had happened to Harry. His initial reaction could be a stunned silence as he tries to process the information.
They could view him as a potential recruit. If Harry has been abused, they might think he could be turned against those who he was supposed to be with, like the good wizards. They would see his abuse as a way to break his loyalty to the side of good and bring him into their dark ranks. For example, they might offer him power and protection from his abusers in exchange for his allegiance.
He'd be angry. Snape is not one to tolerate such immoral and inappropriate content. He might take steps to distance himself from it and try to make sure others in the wizarding world are aware of how wrong it is.
Bill has several options. He can create a blog post or a vlog (if he's comfortable with that) about his discovery and how it goes against the values of the fan community. He can also approach the more influential members of the fan community and ask them to support his cause in condemning this kind of fanfiction. Additionally, he can participate in fan forums and use his discovery as an example to promote a code of ethics for fanfiction writing within the community.