Hi friend,
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 Self Monitoring?
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
Take care
Wishing speedy recovery
God bless
Good luck