On the other hand, it could also strengthen some of his friendships. For example, Hermione might support Sirius's decision and help Harry see the reasons behind it. This could bring her and Harry closer as she tries to keep him occupied with studying while he's grounded.
His friends might not understand at first. They could be confused as to why Harry is being grounded and might try to convince him to break the rules. This could create some friction between Harry and his friends if he refuses, as they think he's being overly cautious.
It could potentially strain the relationship. If Sirius misinterprets the situation, it might create some tension. Harry and Hermione would feel like they need to explain themselves, and Sirius might be a bit standoffish until he gets the real story. For instance, if he thought they were hiding something from him, it could make things a bit rocky between them for a while.
It can strain their relationship. If the argument is really intense, they might not trust each other as much immediately afterwards. For example, Harry might start to question Sirius' motives and Sirius could feel that Harry doesn't respect his experience.
Some of Harry's friends might be initially shocked. Ron, for instance, could be quite skeptical and voice his concerns about the age difference. Hermione, on the other hand, might be more open - minded at first, but as she sees how the relationship impacts Harry's studies or his role in the fight against dark forces, she could start to have doubts.
Their talk could also enhance Harry's magical knowledge. Sirius could teach him spells or magical techniques that he hadn't learned at Hogwarts yet. This new knowledge would be very useful for Harry in his fight against the Dark Arts. Moreover, learning from Sirius would make Harry more independent in his magical growth as he gets advice from someone outside of the school environment.
One common plotline could be Sirius teaching Harry more about his family's history in a way Dumbledore didn't. For example, Sirius might share hidden secrets about the Potter family magic that Harry was never aware of.
In a fanfiction scenario, they could meet in Sirius's old family home at Grimmauld Place. Harry and Ginny might be exploring the place when Sirius appears. Sirius would greet them with his characteristic charm and warmth. He'd probably offer them a drink and then start a conversation about the current state of the wizarding world, asking Harry about his progress at Hogwarts and Ginny about her experiences in the Gryffindor house.
It could enhance their relationship if the fanfiction gives Harry new topics to talk about. For example, if the fanfiction is about a magical adventure similar to their own experiences, they can bond over it.
In some fanfictions, Harry's friends might take on more of a protective role. They see how broken Harry is after Sirius' death and they become more vigilant about his safety. They might also encourage him to open up more about his feelings. Harry, on the other hand, could initially resist their efforts but eventually come to appreciate their friendship more as he realizes they are just trying to help him through this tough time.
If Sirius hits Harry, it can initially strain their relationship. Harry may feel hurt and confused, especially if he trusts Sirius. It could lead to a period of estrangement.
It's possible that in the cave, they start to see each other as equals rather than just a mentor - student relationship. They might argue about the best way to deal with the cave's mysteries at first, but through that process, they learn to value each other's perspectives. Also, if one of them gets injured in the cave, the care and concern shown by the other would enhance their relationship.