Why the fuck didn't he just go to the bathroom? It's THE perfect place to unwind, and there's no chance anyone would find out that he didn't need to go.
Her only purpose seems to be to take him as far away from Chisaki as she can. You can tell that the two others enjoy spending time with him (regardless of how weird their reasons for that are), but Kanako clearly feels nothing.
From context clues she seems to hold a generally low opinion of men. Not only that, but she's a confirmed bisexual who appears to harbor feelings for Chisaki. My guess is she gets off making Ueno dance in the palm of her hand like a puppet, while trying to keep him away from Chisaki without letting her know.