After having given the poor ex-deadra possesed guy the proof or your killing of the witch(didnt kill her she gave me her broom) the quest ends even though the original questgiver should be the one who ends it and says that you completed it otherwise it looks weird if she just gives you a new mission as if nothing happened.

EDIT: Based on the discussion on Discord there is a solution of letting her say something like "Thanks for curing Theodus, now..." when asking her about orders.