Well, Bill could be shocked at first. But then he would likely become very protective. He may take Harry to a safe place, like the Burrow, and gather his family around to support Harry. He would also probably try to find out more about the abuse, like who was behind it and for how long it had been going on. He'd be determined to make sure it never happened again.
He could start teaching Harry some defensive magic in case the abuser tries to come back. Also, Bill might use his connections to find a healer or a counselor who can help Harry deal with the emotional trauma of the abuse.
Well, in many cases, Bill might try to rescue Harry from the abusive situation. He could confront the abuser or find a way to get Harry to a safe place. Maybe he'd also offer emotional support to Harry, like listening to his story and reassuring him that he's not alone.
Well, it depends on what you consider 'good'. There are many fanfictions in this genre, but quality can vary greatly. You might want to check some popular fanfiction websites and look for ones with high ratings and positive reviews.
In the fanfic 'Malfoy's Revelation', Malfoy is paired with Harry for a project. As they spend time together, Malfoy notices some strange behaviors from Harry, like flinching at sudden movements. After some snooping around, he discovers that Harry has been abused. This new knowledge makes Malfoy more empathetic towards Harry and he tries to help him in his own way, which includes standing up for Harry against some of their mutual enemies at Hogwarts.
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.
Well, first of all, the students would probably be shocked. His friends, like Ron and Hermione, would be extremely angry at the Dursleys. They might try to comfort Harry more often. And the school itself could start providing special resources for Harry, such as counseling or extra protection spells. Also, there could be a big change in the way the wizarding community views Harry. Instead of just seeing him as the 'Boy Who Lived', they would see him as a victim who needs support.