Benjy'll know Xeno for sure, what with being in the same house and having to listen to Xeno's nonsensical ramblingsdiscoveries. Benjy will humor him and be friendly, even if he doesn't actually believe Xeno (but there's always a chance, right?).
And Mary will know him from classes, at least. She'll probably roll her eyes at him and has made a point of joking along with him (even if he thought she was serious) about snorkaks and the like. But she knows he means well and thinks he's not so bad.