Well, McGonagall is a very caring and strict character. When she discovers Harry is being abused in fanfiction, she would first ensure Harry's safety. She might offer him a safe haven at Hogwarts all year round if possible. Then, she would start an investigation into the abuse. She'd use her influence and connections in the wizarding world to get to the bottom of it and make sure justice was served for Harry.
Once McGonagall discovers the abuse, the story could take many turns. Harry could be given special training or mentorship by McGonagall to build his self - esteem. The wizarding world might also change some of its policies regarding the protection of children in response to Harry's case. Meanwhile, the relationship between Harry and McGonagall would grow stronger as she becomes a sort of surrogate family for him, and he would rely on her for guidance and support in his journey of recovery from abuse.
She might be horrified and immediately take steps to protect Sirius. For example, she could confront the abuser or report it to the proper authorities within the wizarding world.
They might face magical punishment. For example, they could have their magic restricted or be sent to Azkaban if it's a really bad case. McGonagall would not tolerate such behavior and would push for the strictest punishment.
One great fanfiction in this genre is 'A New Beginning' where McGonagall steps in to rescue Harry from his abusive relatives. It beautifully shows how she nurtures him and helps him grow in confidence.
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.
The other students could have a range of reactions. Some might be sympathetic and try to befriend Harry more. Others might be afraid, especially if the abuser is still around. But overall, it would create a stir in the wizarding community, and there would be a general outpouring of concern for Harry's well - being.
The abuser might face the wrath of the DA. They could be ostracized within the wizarding community. The DA members could use their influence to make sure the abuser is held accountable for their actions.