Dung would normally never notice a little firstie like Marlene McKinnon.
She came up to him in the library, though, and asked him what was wrong.
He should have told her to bug off. Instead he explained that he was deciding whether or not to drop out of Hogwarts, now that he had his OWLS.
She was adorably earnest: "Education is a precious thing, but I'm sure you'll be a wonderful wizard whatever you do."
He didn't see her again until years later. Then it was only a picture in a week old newspaper.
She and her entire family were dead.
When the Potter boy started saying that Voldemort had returned, Dung planned to lie low. It was stupid to get involved with the war against the Dark Lord. Look what had happened to Dorcas Meadows, to the Prewett boys… look at what had happened to Marlene.
I'm sure you'll be a wonderful wizard.
The next day he went to Dumbledore, and asked to join the Order of the Pheonix.