According to the H2P for the Inquisition Leader:
The Inquisition Leader is not able to recruit any player who already has a night-faction (Familiar, Coven member, Undead member, Mason, Illuminati Member) or any user of Witchcraft but will not be killed if they try. This failed recruit does not count towards the ability use limit.
The recruited villager retains their original ability.
And for the Graverobber:
The Graverobber is not seen as a user of witchcraft.
Therefore, it makes sense that the Inquisition Leader can recruit a Graverobber, who can then go on to steal the role of a village witchcraft user.
If the Graverobber successfully steals the role of a village witchcraft user (e.g. seer) will they remain part of the Inquisition? If so, do the Inquisition need to kill the ex-Graverobber in order to fulfil their win condition?