After a couple of years, she started dating a man who began talking to her in a demonic voice. What caused this?

Here again, is another example of being around someone who is possessed by spirit meddlers. While this is true of 90% of people, not all are influenced to the same degree to be dark in nature. Some have periods of time where the spirits gain more direct control of things and this can change the person's demeanor, influence their thoughts, feelings, and actions, and even spur planful criminal and violent activities at the extreme. So this is a brush with evil she was able to perceive and pull away from, in being quite a different vibration, and able to sense something deeply wrong with that person and that was a godsend, literally.