What can I do if my mother fell in the parking area of a medical facility because it was very dark out there and the sidewalk was not apparent?

IF the parking area was not lit to a reasonable degree--basically, to the level that other commerical venues typically light their parking lots--then that may have been negligent (or unreasonably careless) and your mother might have a cause of action or legal claim against them. However, if the lot was more-or-less lit to the generally acceptable level for parking lots, then the facility probably did nothing wrong and would not be liable.

But even if the faciltiy might be liable for not having adequate lighting, your mother would have to have medical evidence (e.g. tests, a diagnosis, etc.) linking the fever to the fall--without a causal link, she could not recover for the fever. And even if there was a causal link, it's not clear that she could recover enough money to justify a lawsuit, since what you can recover is linked to the medical costs you incur, wages or earnings you miss or lose (if any), and the severity and duration of the impact of the injury on your life. Sinply having occasional fever, without more, would not support much compensation.

Since the most important thing is to take care of health first, then worry about any lawsuits, your mother should see a doctor about the fever and find out what is causing it, what the effect on her life is, whether it's a symptom of a greater problem, how to treat it (and how long and how costly the treatment will be), etc. Once she knows that (and has dealt with any immediate health risk), she should take the medical information to a personal injury attorney to discuss the matter in detail and see if she has a viable case. Good luck.