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.
Bill would take several important actions. First, he'd provide a safe and stable environment for Harry. This means making sure Harry has a comfortable place to sleep, enough food to eat, and people around him who care. Second, he'd try to boost Harry's confidence which has probably been shaken by the abuse. He might do this by taking Harry on small adventures or introducing him to new friends. Third, Bill would look into the legal or magical regulations regarding abuse in the wizarding world. He'd want to make sure the abuser is held accountable for their actions.
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 probably gets angry. Bill might be furious at the person who is abusing Harry and vow to protect Harry at all costs.
Amelia Bones might confront the abusers directly. If it's the Dursleys, she would give them a stern warning or even use magic to make sure they can't harm Harry anymore. She could also arrange for Harry to have proper magical education away from the negative environment he was in, so he can grow and develop his skills without fear.
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.
He may report the abuse to the relevant authorities in the fanfiction setting.
Cedric might first talk to Harry privately to get all the details. Then he could go to Dumbledore for help.
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.
One consequence could be that Bill feels a strong sense of protectiveness towards Harry. He might immediately try to comfort Harry and ensure his safety. For example, he could offer to let Harry stay with him where he can keep an eye on him.
Well, Bill might be really surprised at first. If he's a big fan of the original story, he could be either intrigued or a bit put off by the fanfiction. He might start reading it out of curiosity to see how others are imagining Harry and Ginny's relationship in different scenarios.