Following your gathering the blocks for the boy in Rupert's flat you learn that his wife is the zombie in the basement. There is nothing for it - she has to be dealt with - and Rupert lacks the heart to do it so you have to!
When you get downstairs you need to go to the end of the basement and turn off the live current, and then go through the hall that was electrified and open the room that she is locked in and kill her.
Once you do that return to Rupert and complete the quest.