Welcome to Health Care Magic
You sound scientific and intelligent indeed. As you surmise, it is most likely Postural Hypotension
. It is easy to confirm - just check her pressure at the two positions! You can measure it at home - if you have the machine.
It is unlikely to be Diabetes
- 'only symptom and only on standing' can not be accounted for. However, for her satisfaction, do a blood sugar - she can be convinced immediately! I presume, you could test at home? - Are you not doing Home Glucose
The body's balancing mechanism is in the inner ear
. An ENT (Ear Nose Throat) specialist is the one to see. They have special tests to confirm or exclude the contribution of the ear - which probably is the case with you.
If there are no clues, you have to be followed up by a neurologist
ECG, blood counts and other investigations may be needed
Wishing speedy recovery