To point out that he was the one who started it probably wouldn't win her any points. He was the one who had gotten short with her after she'd tried to keep herself in line. Lucy ignored his insults -- she'd been called much worse, but had long since learned to steel herself against them. They were just words, and she honestly and truly believed she was doing the right thing.
"Was that so hard?" she asked, trying not to flinch when he pushed himself so close to her, looking and sounding so pissed off. "If everybody just walked away instead of picking up after themselves, we'd be shuffling by knee-deep in trash." Her voice was filled with conviction, maybe just a touch preachy. By now, he was already starting to move away from her; the cup had been thrown away, so that was good, but she didn't think he quite got the message.
"I just think you should be a little bit more careful, a little bit more aware. It makes the city better for all of us." She was heading in the same direction he was, anyway. There wasn't anything that said she couldn't continue to talk to him while she headed home.