Harry automatically reached out before he knew what he was doing, when he saw Zach stumble. He caught hold of the blonde's sleeve to keep him from falling, and almost as quickly let go when he realized what he'd done; he didn't want it to seem like a hostile move.
And yet again, he felt the urge to reach for the stone that he'd been carrying around in his pocket. But it wasn't there anymore, he couldn't have used it right now even if he wanted to. He'd have to go back to Godric's Hollow and dig it up, but maybe-
No. He knew that was a bad idea, even as he thought it. That stone was dangerous, and he couldn't knowingly put anyone at risk of using it badly. Not Bill, not Fred, not any of the people he loved and trusted who had lost their loved ones; he wouldn't do for Zacharias Smith what he hadn't done for them. He'd hurt the man in front of him enough, and he hadn't even done that on purpose.
But what Zach was saying - that wasn't his fault, that Cedric hadn't known Zach loved him. If he should have seen anything weird about that, Harry felt too guilty to realize it, and he wasn't mean enough to point out the part that wasn't his fault.
He searched for words; found none. Luna had known what to say to him when he was grieving, but he wasn't Luna. He stood there for a long moment, feeling useless, and then offered, "I was going to go and, er, have some lunch - do you want to come with me?" He tried to muster a smile, bravely, though it didn't reach his eyes. "You can yell at me more, if it'd make you feel better."