The Hashiras would likely show concern. Some, like Shinobu, would immediately start looking into possible medical solutions. Giyuu might be more reserved in his worry but would still keep a close eye on Tanjiro. The other demon slayers would also be affected. They look up to Tanjiro, so his sickness would lower the morale of the group. They would try to support each other while waiting for Tanjiro to recover.
Tanjiro might use his sense of smell to detect hidden threats. Since his sense of smell is super - enhanced, he could sniff out any demons or traps in the way of saving Rengoku. For example, if there are demons disguised as allies, Tanjiro would be able to tell by their scent.
Some friends were likely horrified. The Chris Watts case was so heinous. His actions of murdering his family were unimaginable. Friends might have initially been in disbelief, thinking that the person they knew couldn't be capable of such a thing. They could have felt a sense of betrayal as well, since they thought they knew him but he had this dark side.
Draco's Slytherin friends would be quite shocked. Some might shun him, thinking he's betraying the Slytherin ideals. However, a few might see the advantage of having a connection to Harry Potter. For example, if there are any threats to Slytherin as a whole, Harry's influence could be useful. So there would be a split in how Draco's Slytherin friends react.
Yes, in some fanfictions, Tanjiro is depicted as the reincarnation of Yoriichi. This concept adds an interesting layer to the story as Yoriichi was an extremely powerful and legendary figure in the original lore. The idea allows for new storylines where Tanjiro might inherit some of Yoriichi's skills or have a deeper connection to the past events related to Yoriichi.
Tanjiro's fate in the manga is that he ultimately passed away. This event was a significant part of the story's development and had a profound impact on the other characters and the overall plot.