Well, James is a loyal friend. So, when he discovers Sirius is abused, he would most probably drop everything else he is doing to focus on Sirius. He'd be angry, not just at the abusers but also at himself for not realizing it sooner. He'd make sure Sirius knew that he wasn't alone and that he had someone who would always be there for him. James might also encourage Sirius to learn self - defense magic more intensively so that he could protect himself in case the abusers tried to harm him again.
James might be shocked at first. He could then feel a sense of protectiveness towards Severus. Maybe he starts to see Severus in a new light and tries to find ways to help him, like keeping an eye on those who abuse Severus.
Sirius' behavior might change. He could become withdrawn or overly jumpy. For instance, he might flinch at sudden movements or loud noises. James, who knows Sirius well, would pick up on these unusual behaviors and start to investigate, which could lead him to discover the abuse.
Sirius would likely be extremely angry. He already has a protective streak towards Harry. He might immediately confront those he suspects of the abuse, whether it's the Dursleys or others at Hogwarts. He would probably also try to remove Harry from the situation as quickly as possible.
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.
Remus is likely to be shocked and angry. He might immediately want to protect Sirius. He could confront those who are abusing Sirius or try to find a way to get Sirius out of that situation.
After James discovers Severus' abuse, Severus may experience a range of emotions. He might be initially reluctant to accept James' help, given their history of animosity. But if he does accept, it could lead to a transformation in his self - esteem. He could start to believe that he is worthy of protection and care. In terms of his relationships at school, it could either improve his standing if the other students follow James' lead in protecting him, or it could make things more complicated if there are those who oppose James' new - found support for Severus. His academic performance might also be affected. If he feels more secure, he could focus better on his studies, but if he is dealing with the fallout of the abuse being known, it could be a distraction.
The abuser could face magical justice. In the wizarding world, there are probably spells or enchantments that can be used to punish wrongdoers. James Potter might use his connections to ensure the abuser is dealt with severely, like being sent to Azkaban or having their magic restricted.