I have had this problem ever since I can remember (early childhood sexual abuse, long-term). When I was a kid I always felt someone behind me, under the bed, behind the shower curtain, *behind me in the shower,* standing right outside a second-story window (craziness!), my closet--everywhere. I was afraid to pull my shirt over my head or towel dry my hair, because I was certain when I could see again there would be a man right behind me. I regularly become so convinced of these delusions that I will bolt somewhere to the point where I trip and fall in fear.
I have always been afraid of being outside alone or in a group of only females or a male I consider incapable of protecting me (very frustrating for some of my male friends, but I can't help it!). I used to hide from every passing car, or walk up to someone's door randomly because I thought someone was following me, and I wanted to throw them off and make them think I lived somewhere I didn't.
I have worked a lot on these symptoms and am now confident in being outside alone during the daylight, but this change happened over a long course of time and took living at several extremely safe environments verrry far away from where my traumas occurred. The first time I felt comfortable walking around in daylight was in a kibbutz in Israel, a small socialist/communist community that generally focuses on sustainable farming and peaceful communal living. The second was at school, an extremely small 'work college' where everyone knows everyone and most people are pacifist hippies who like to work hard.
My only weakness in these environments have been parking lots. A counselor once suggested that something happened to me in a parking lot. This completely bewildered me until I began to think about it closely and found that yes, a string of things did occur to me in parking lots, though they are not the things I usually think about as the forefront of my trauma, though parking lots had a lot to do symbolically with the major trauma I experience. I suppose I created an association between parking lots and the abuse I suffered. Now when I am in parking lots I experience hyperviligance the worst, even if I am in a locked car.
Perhaps your sufferer could try to identify some of the specific situations where she feels most threatened and try to free associate around that event. What went on before she felt threatened? Where was she when she felt threatened? Do her followers have faces, or are they just shadows, or both (I have both)? Where do those images come from? Do they remind her of something? What was the reality of the situation? What perspective may have been helpful while she felt threatened? What activities? I haven't heard these straight from a therapist, but these sorts of questions have helped me learn about myself and make progress on my hypervigilance.
Sorry for so many words; this is the only way I can express myself. I hope it's helpful.