This was framed as "terrible accident" in some of the older chapters, but honestly nothing there screams terrible (except for the girls psycologies). She didn't drown if the other boy rescued her within minutes, as people don't drown in seconds, they drown in minutes. Now the blood loss (or more likely the shock) from the wound could make her loose consciousness, but again it doesn't seems more then a scratch.
It could be that the grandpa framed the incident as way more serious then it was, framed Long as a victim of the incident too (seeing Long in the hospital bad trigger a lot of suspicions), and manually made them unconscious (or for hudai kept her unconscious even if she would be perfectly able to wake up), so both were admitted to the hospital (despite the risk of having a girl manifesting scales in an damn hospital). Also probably he needed them both out for the memory wipe. The scope was probably to push huadai parents away, probably becouse while he hoped for huadai and long relationship to start anew, he thought that both should be more mature, as he probably thought that what happened was a sign they weren't ready, so he wanted her out of the town.. This despite he knew he would risk not seeing any of them again. He did probably hope she would have choosen to return when she would have been indipendent from her parents, but he died before, and after his death she probably wouldn't have a reason to return (until the debt). Also the parents probably tryed to indoctrinate her on how the grandpa was bad.
I wonder what he said to her parents, becosue that would change my idea of them (from assholes to idiots). The only things we know is that they say he had dementia (or similar), but it's not clear whetever they actually believe it. If they believe it they are assholes, as a person with dementia (speaking broadly dementia is a term encompassing various patologies, let's restrict to the ones involving strictly the mind (personality, memories, lucidity of thought) ) is not responsable. LEaving a demented relative alone is one of the most shitty thing a family member could do. I could understand if he was directly dangerous (the disease make him violent, or it was a dangerous person even before). Granted avoiding the 2 to spend time alone again would have been a sensible choice. But making a young girl flip her entire world becouse of this seems idiotic.
Unless he told them he was directly responsable, but even that what he told them could change the frame.
I should reread the scene with her parents, but I don't recall them explicitly saying her accident was caused by the grandpa, so I don't think is that.
Last option is that the grandpa itself told to them to leave the town, maybe scaring them with risking hudai safety. But then why trashing him with all the dementia talk? They could have easily said to Hudai "he demended we left town".
I don't honestly think the parents know the actual truth.
BOth in case of dementia or accident the parents are assholes. That spot was clearly a know playground for children. An incident happened, the affected got a scar (also the scar that was shown before is different that the one showed here, so it's possible that was left isn't the scar at all), it's not the end of the world. I have some friends with quite the big scars it's not like they totally excluded people from the same context they got the scar, becouse they weren't the result of direct actions.
Second option is the more understandable for the chain of action. But unless he framed himself as directly invovled, and an active danger to her safety , cutting contact would have been normal, but even leaving town? (town itself doesn't seem that small)
Last options is the one the parents result as less assholish. They are still idiot, and still a bit of assholish, for refusing to clearly thell hudai this things (unless not telling hudai was part of what gran told them, to not possibily nullify the possibility of her return) and for costantly trashing the granpa.
Yehah I hope next chapter give an proper explaination on this. As I may be overreading (and the author maybe wanted to clearly show hudai parents were assholes)