I can chime in on this. I personally have this problem for many months. While it's easy to say that we don't have any stress/stressful problems in our life. A lot of these problems are not always obvious to us. For me, it's as simply as worrying about not be able to sleep.

Also, there's no 'normal' insomnia. Not being able to sleep is not normal. Lack of sleep will create a vicious cycle.

Lack of sleep > Stress (worried about not being able to sleep) > Anxiety (became worse) > Lack of sleep

Try to figure out what form of problems that she's going through in her life. If there's REALLY NONE. Then she need to seek specialized doctor. Try find doctors that are specialized in sleep.

Things that worked for me or I tried

1. Vitamin D3
2. Melatonin (though I double the dose and may or may not work sometime)
3. Avoid eating 2-3 hours before bed
4. Shower when it's time to sleep.
5. Dim all lights around her. Especially phones and monitors. If she REALLY need to look at those device. Download Twilight app for her phone and Flux for her computer.
6. Exercise. (Though this technically didn't work for me as I lift weights and weight lifting is a form of stress if rest is insufficient)
7. White noise, it depends from person to person. For me, any form of distraction is not good as I'll be focused on the distraction... wide awake
8. The best solution for me, alcohol. I happened to have a lot of whiskey at my disposal, so I tried. And it works  wonder. Alcohol fogs your mind from all those troubling thoughts. I don't recommend downing a lot, I usually just pour a very small amount + ice and sips slowly.

Also, if her job is quite demanding or relatively busy, have her to take a few days off and go to a place where she can relax.