One reason could be Snape's sense of justice. Despite his often - cold exterior, he may not tolerate Ron's unjust behavior towards Hermione. Another reason might be that he sees something in Hermione that he values, like her intelligence or her thirst for knowledge, and doesn't want it to be extinguished by Ron's actions.
Well, it could be that Snape has been secretly observing Ron's erratic behavior towards Hermione. When the situation gets out of hand, he intervenes. He might use his position as a teacher to discipline Ron and in the process, saves Hermione. Snape is a complex character with a lot of power and knowledge, so he could use various means like his occlumency skills to sense the danger Hermione is in and act accordingly.
One reason could be Snape's sense of justice. Despite his often - cold exterior, he might not tolerate Draco's bullying behavior towards Hermione.
Snape could be the voice of reason. He might see the potential chaos and damage that a romantic relationship between Ron and Harry could cause, not only to their friendship but also to the whole Hogwarts dynamic. So he steps in to prevent it.
He could use his position of authority. Snape might confront the abuser directly. Since he's a teacher, he has the power to intimidate and put a stop to any immediate harm. He could also give Hermione a safe place to stay, like his own quarters (if it's within the rules of the fanfiction world).
Charlie might be a character who has known Hermione for a long time, maybe through some other connections in the wizarding world. He sees Hermione as someone in need of protection from Ron, whether Ron is acting out of his own accord or is being influenced by external forces. In relation to Ron, he is the one who stands against Ron's actions towards Hermione.
There might be some kind of misunderstanding. For example, Ron could have been misinformed about Hermione's actions. He thought she betrayed him in some way, like sharing Gryffindor's secrets with Slytherins, so he lashes out at her.
One consequence could be that it creates a rift between Ron and Hermione. Hermione might be grateful to Lucius, which could make Ron jealous or angry. This could lead to a strain in their relationship, and they might have to work through their feelings about Lucius' interference.
Perhaps in the fanfiction, Cormac invites Hermione to a secret gathering, but his intentions are not so noble. Ron finds out and races to the location to save Hermione. This act of heroism by Ron might make Hermione question her feelings for him. She might start to see him as more than just a friend, especially if Ron shows a side of himself that she hasn't fully appreciated before. The story could then follow how their relationship evolves from this event.
In this fanfiction, it could be that Ron is under the control of a powerful wizard who wants to get to Hermione for some reason. Charlie discovers this plot and decides to protect Hermione. He sets up various traps and wards around the place where Hermione is staying. When Ron, being controlled, tries to attack Hermione, Charlie is there to intercept him, engaging in a magical duel if necessary to save Hermione.
Well, in some versions, Ron might use his quick wit and knowledge of Hogwarts' secret passages to outmaneuver Draco. He could lead Hermione to safety through a passage that Draco doesn't know about.