Lily had been watching the bloke in the chair, and she'd reasoned it was only a matter of time before gravity won out over his impressive balancing skills. She wasn't surprised when he fell, and if she hadn't been in a place crowded with Muggles, she would have at least tried to throw down a cushioning charm.
As it was, she'd simply ensured nothing of her was in the way when he did fall. "No worries," she assured him when he apologized. "You're all right then?" she asked quietly as she regarded him. Why not? Her book could only fascinate her for so long and she never had been the sort to want to be completely alone. So why not talk to a stranger for a bit, if he was inclined? She hadn't missed that he had friends with him, so perhaps this was only going to be a brief chat.
Either way, it was interesting. She glanced around, but most everyone seemed to be returning to what they'd been at before, with the occasional gesture and grin in the bloke's direction.