Kakashi is very much alive in the Boruto manga. The manga doesn't include his death. Maybe you got some misinformation or confused it with another character's fate. But for sure, Kakashi is around.
Kakashi's reaction would be complex. He is a man who has lost many people in his life. But Naruto was special. He would hold Naruto tightly, his hands trembling slightly. He might try to use some of his own chakra to keep Naruto alive a little longer, but deep down he knows it's futile. His face would show a mixture of pain, regret, and love. He would remember all the times he trained Naruto, all the lessons he taught him, and how Naruto had surpassed all expectations.
Kakashi might immediately offer a comforting presence. He could put his hand on Naruto's shoulder and just be there silently, letting Naruto know he's not alone.
At first, Kakashi might try to distance himself from Team 7's possessiveness. He values his independence and doesn't want to feel smothered. However, as he realizes that their actions are coming from a place of deep love and loyalty, he starts to accept it more. He may even start to rely on them a little bit, for example, allowing them to help him with his paperwork or other small tasks that he usually does alone. This shows a new level of trust and closeness between them.
In fanfiction, Kakashi might comfort Naruto by simply being there for him. Just sitting in silence, showing Naruto he's not alone. For example, they could be on the Hokage Rock and Kakashi would put a hand on Naruto's shoulder as they look over the village. It's a silent yet powerful form of comfort.
In this fanfiction, Kakashi could help Naruto in his training through mental conditioning as well. Naruto is often impulsive, so Kakashi may work on his patience and decision - making skills. He could use his Sharingan (if applicable in the fanfiction) to analyze Naruto's movements during training and give him detailed feedback. Moreover, Kakashi could introduce Naruto to different types of enemies and battle scenarios in a controlled training environment, which would broaden Naruto's understanding of combat and make him a more versatile ninja.
Well, Kakashi is a complex character. When he discovers Naruto is abused in the fanfiction, he may first feel a deep sense of guilt for not noticing earlier. He'd then use his skills and influence in the village. He could start by reporting it to the proper authorities in Konoha, like the Hokage. And at the same time, he'd be there for Naruto, providing emotional support, talking to him, and helping him through the trauma. Kakashi has his own past traumas, so he'd understand what Naruto is going through on some level.
Kakashi would be deeply concerned. Given his relationship with Sasuke, he'd feel a strong sense of duty to save him. He'd use his extensive network of contacts within the ninja world to try and find out where Sasuke has been taken. He might also reflect on his past training with Sasuke and how he can use that knowledge to anticipate the kidnappers' moves. He'd be ready to face any danger or obstacle in his path to rescue Sasuke. And he'd be constantly thinking about Sasuke's safety and well - being during the whole process.