Sirius. Oh, that boy needed smacked 'upside the head' for that. A PRANK? Betraying a friend's deepest secret that could have ultimately led to that friend's expulsion--and possibly his death? If Snape had died, I can't think that even Dumbledore could have saved Remus--and Remus would never have forgiven himself. It was completely thoughtless and cruel. Sirius is lucky that Remus ever talked to him again!