Hermione might seek revenge. Given her intelligence and magical abilities, she could use her skills to get back at Harry and Ron for their betrayal. This could lead to a big rift in their friendship and even endanger their common goals in the wizarding world. She might also side with other characters who are against Harry and Ron, changing the whole dynamic of the story.
At first, Harry might be confused. He would try to figure out what went wrong. He might search his memory for any signs that could have led to this betrayal. Then, he would likely experience a range of emotions from sadness to rage. Depending on the nature of the betrayal, he could either try to win them back by showing them the truth or decide that they are no longer worthy of his friendship and focus on his own path, maybe even leaving the wizarding world for a while to clear his head.
It could be that a prophecy reveals something that makes Ron and Hermione think Harry is a danger to the wizarding world. They might decide to betray him in order to protect the greater good. However, as time goes on, they could realize their mistake. But by then, the damage to Harry's trust would already be done, and he would have a hard time forgiving them.
Perhaps Ron was influenced by outside forces. There could have been dark magic or some other characters manipulating him. It's not uncommon in the wizarding world. Some powerful dark wizards might have preyed on Ron's insecurities to make him turn against Harry.
Sometimes it's to show a conflict in their relationship. Ron and Hermione have a complex relationship, and a slap can symbolize a breaking point in their interactions.
One popular one might be 'The Moment of Impact' where the slap is a turning point in their relationship. It delves into the emotions and misunderstandings that led to that moment.
Jealousy is a big one. Ron often feels like he's in Harry and Hermione's shadow. He might see their achievements and feel left out or unimportant, which could lead to his betrayal.
In some fanfictions, it could be a misunderstanding that spirals out of control. Hermione and Ron misinterpret one of Harry's actions. For instance, Harry might be trying to protect them in a secret way, but it looks like he's up to something bad. Their lack of trust then grows, and they end up betraying Harry, perhaps by not coming to his aid when he really needs them, which could have serious consequences for Harry.
Well, it could be that in these fanfictions, a dark influence overcomes Hermione and Ron. They might be manipulated by a powerful dark wizard who makes them believe that betraying Harry is the right thing to do. It could also be a miscommunication situation where they think they are helping but end up betraying Harry. For instance, they might be tricked into believing that Harry has turned bad and needs to be stopped.
Yes, there are some out there. One example might be on FanFiction.net. You can search for it using those keywords and look through the reviews to find a well - written one. However, different people have different ideas of what 'betrayal' means in this context, so you might need to read a few to find the one that suits your idea.