Well, maybe it happens when he accidentally walks into a girls' changing room or sees some laundry.
In the fanfiction, if Ron finds out Harry is abused, it's a complex situation. Ron's reaction would be multi - faceted. Emotionally, he'd be a mess. He loves Harry like a brother. He might feel guilty for not noticing earlier. Ron would also start to view Harry in a new light, with more compassion and understanding. He would use his Weasley family connections to find resources or help for Harry. Maybe he'd ask his mother for advice or even get his older brothers involved if the abuse was from a powerful source within the wizarding world.
It has a negative impact. It makes many fans who respect the original work feel disgusted. The Harry Potter community is built on the love for the original story's values like friendship, courage etc. This kind of inappropriate fanfiction goes against those values and can create a rift within the community.
Harry might react with shock. If he overhears something unexpected like Ron and Hermione confessing their love for each other, it would be a big surprise. He might stand there frozen for a moment, not knowing how to process it.
Ron might be shocked at first. He could become very protective of Harry, standing up for him against any potential threats. Maybe he would also be a bit angry that he hadn't noticed earlier.
Ron could also be really happy for them. In certain fanfic scenarios, he might have noticed the chemistry between Hermione and Harry all along. So when Hermione asks Harry out, he's like the biggest cheerleader. He might offer to cover for them when they go on dates or give Harry advice on how to make Hermione happy. He would see it as a natural progression of their friendship and be glad that they are exploring a romantic relationship.
It can divide the fandom. Some fans who love the original dynamic between Harry and Ron might be disappointed or even angry at this deviation. They might see it as ruining the essence of their friendship.
Maybe he uses some powerful magic spells that he has learned secretly. For example, the Patronus charm could be used to fend off the Dementors which are the main guards in Azkaban. He might also get some unexpected help from his friends or some mysterious characters within the fanfiction world.
It could also influence other fan works. If this type of fanfiction becomes popular, it might inspire more works that explore negative relationships between the main characters. This could change the overall tone of the fan community's creations, moving away from the positive and friendly relationships that are often emphasized in the original books.
Maybe someone who witnessed Regulus' death tells Sirius the truth. This person could be a fellow Order member or a magical insider.
You can find a lot of 'Harry Potter Ron and Harry Fanfiction' on Fanfiction.net. It has a huge collection of stories related to the Harry Potter universe, including many that focus on Ron and Harry's relationship.