Where Su found frustration in the fact that she couldn't touch anything, Kevin got extremely distracted by the fact that he could pass through things. He watched his hand go through stone walls and trees; he slid into Honeydukes and dived into a barrel of Chocolate Frogs, seeing them squirming inside their packages when he angled his face just right so his eyes were inside the package. It would be cool if he could feel them squirming on his arms and face, but he wasn't too bothered by it.
He was heading somewhat in the direction of Hogwarts, though not particularly aiming for it; he didn't know where anyone was, so he hadn't chosen a particular direction. It was just the instinct to explore that guided him, and the knowledge that he would eventually stumble upon someone.
And there, suddenly, in the distance, he saw a shape. A ghostly shape, much like his own. It glimmered in the evening light and he found it hard to see well, but he floated forward as fast as he had figured out how to move, and came up to them.
"Su!" he exclaimed, when he'd gotten close enough to see her. He reached out, and - to his very great surprise - actually touched her shoulder. "Su! Oh! You're a ghost too!"
It occurred to him that he ought to be sad, but he wasn't. He couldn't find it in himself to be sad for his own death, when he didn't feel dead; he felt ghostlike and awesome. Then it occurred to him that to become a ghost, Su must have died, probably from that flash of light that had come at them, while his eyes were fixed in terror on the charging spider. She didn't look even remotely glad about it, so he hesitated before asking a little bit plaintively, "Isn't it cool?"