Does she get dizzy/lightheaded when she stands up? Ever fainted or came close to fainting? And how does she feel if she has to stand in place for a while?
Has she seen a cardiologist or neurologist? Blood sugar thoroughly tested? Thyroid?
Lots of questions, I know.
I have low blood volume, low blood pressure, and neurocardiogenic syncope. It causes a lot of lightheadedness and other symptoms. My mother shows some symptoms and used to faint when she was younger.
Other conditions that can be hereditary and cause similar type symptoms is mitral valve prolapse.