Any visual field
defect in the lower half of the eye is due to a retinal lesion in the upper retina. the kind of defect you describe may be due to a condition called Posterior vitreous Detachment
or PVD where the vitreous or jelly like fluid of the eye detaches from the retina. Most often, the patient has no noticeable symptoms or just appreciation of floaters
in the field of vision. Another cause of defective field is a shallow detachment of the retina. The causes of these may be myopic refractive errors or, more commonly an age change in the eye. Trauma or an inflammation in the eye may cause PVD as well. There is no contribution of systemic diseases in this condition. Hence, an immediate consult with your family practitioner is not mandatory. Further tests like a B scan , visual fields and perhaps an OCT for the eye may be needed and are probably what your wife will undergo in the next visit.