Apparently he got it farming draksis along with almost everyone else so either he got really lucky or everyone else is really really unlucky. There may be teirs of exotics and you have to be a certain level/light level or perhaps accomplished some prerequisite task prior to getting the drop as of now I'm unconvinced there are exotics linked to certain bosses because that would just promote the kind of exploitation that happened with draksis just with other bosses. I dunno just my opinion.