Seamus' snarky comments aside, Neville knew what he had seen, and he wasn't about to forget about it either. Since there were no Herbology classes that day thanks to the destroyed greenhouses, he decided to spend his hour walking the grounds. He tried to think of places something like that might hide during the day - the old sheds near Hagrid's hut, the caves under the lake... the cemetery.
He saved the cemetery for last since it was further away, but it didn't take long once he got there to see something was wrong, even through the prevailing mist. One of the graves had been disturbed - there were piles of dirt all around and a general mess that suggested a struggle. He broke into a run, pulling out his wand - his stomach turned over as he saw the headstone with Fred's name.
He looked around, half-expecting the perpetrator to rear his head, but he was totally alone. Anyway it didn't require much imagination to figure out who was behind it. His first thought was for Ginny. This would devastate her, he knew.
He fell to his knees beside the grave, staring at the mess. What did he do? Tidy it up? Get help? Should he tell the Weasleys or not?
While all these questions were stabbing almost painfully at his brain, he thought he heard a sound. He looked around - still nothing. Then he heard it again, and slowly he looked down to the disturbed ground below him. "Bloody hell..." he gasped, scrambling back on his rump in horror. "Hello? Is - is someone -?"
He heard it again; a muffled cry. Bole. Of course he wouldn't stop at just desecrating a grave. It wasn't fun unless he murdered at least one person.
He crawled quickly back to the edge of the grave, pulling out his wand - he was sweating despite the cold dampening effects of the mist. "It's okay!" he called out, with no idea if whoever was trapped down there could even hear him. "I'm coming, I'm..."
He cast a levitation spell, managing to lift a Quaffle-sized clump of dirt and throw it aside. He cast it again and again, sometimes having to try the spell twice when he stammered over his words in desperation. He started digging with his other hand, pawing at it for what little good that did, until the hole was too deep to reach. It seemed to take an age. He was just starting to think he should take the time to fetch help, when he saw a sliver of wood under the black dirt.
Frantically he jumped down into the hole, brushing away rest of the muck with his hands. Thankfully it was loose after already dug and refilled once, and it came away easily enough. "I'm coming, I'm coming," he called to the screamer, who he could hear much more easily now, and finally - finally - the lid came loose and he tore it up and aside, leaving splinters in his hands as he did so, but he barely noticed.