Welcome to the site.
Well, it seems like your girlfriend is delusional as well as having hallucinations because what she has been describing cannot be explained with logic and objective reasoning. So it is my opinion that you get her out of the house and put her in a different environment / surroundings because may be the haunting house premise might have affected her mind so strongly that she had developed these delusional thoughts and hallucinations.
So, first thing that needs to be done here is to bring her out of the house and try not to have any sort of discussions with her about the house and its haunting premise. Many times breaking the person's connection with the stressful surroundings can help in improvement in symptoms. Besides this you need to get her thoroughly evaluated by a psychiatrist for paranoia and psychosis because her behavior is showing traits of a psychiatric illness.
I hope this helps.
Wish you all the best.
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