Peter hadn't been thinking of her as a Slytherin in that moment, actually; he almost never did. He simply assumed that, all three of the Potterlets being James's kids, they all had that same predilection for risky and unwarranted behaviour. Unfair at times? Sure, but not down to house prejudice. Such things were best left behind at school, in Peter's opinion.
He stumbled a bit at the unexpected grip on his arm, and grinned at her as he did his best to keep up. "You never know what you'll find at a tree farm," he said. "We should find a lopsided one for your uncle Sirius."