Blood Pressure Abnormality

This is a worrisome problem, in which a person with hypertension (high blood pressure) gets a marked fall in blood pressure when standing (called orthostatic hypotension). You don't mention the age of the person and other potentially contributing health problems, and I am assuming the person is not on drugs.
Such falls in blood pressure are generally due to a decreased output of blood from the heart and a failure of the small blood vessels in the body to appropriately constrict with a change in posture. When the problem is severe, as you describe, one must look for failures related to function of the nervous system. For example, diabetic effects could result in such falls in blood pressure.
The problem you describe calls for a careful history and physical examination by a physician, who can then order studies that may be necessary to solve the problem.
Please be aware that I am unable to diagnose medical conditions online.