IIRC, after I went upstairs to take care of the last guy, I spoke with the girl. She told me to "go downstairs" as well. I did that and she followed me downstairs. She then talked to me and told me she was going to leave, etc. Shortly thereafter...like maybe 2 min. real time...her guardian showed up. You might have gotten stuck in a loop or something.
Might want to back up to a previously saved game. Hopefully you saved before entering the house to start the quest? If you didn't, you should take a lesson here. Always save before kicking off a quest...
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Now where did I leave that doughnut?!