I'm trying to locate the script that makes Ruzka play with the red ball, but I can't find it on her NPC, or on the red ball. Any ideas?

Sorry to disappoint you. But Ruzka "playing" with the ball is coincidence. They (ball and the YaoGuai) aren't persistent references so there can't be any script which influence them. Ruzka is set to a standard Patrol and sandbox package so it's pure coincidence when it bumps in the ball.