The average person measures at about 5 foot 10” inches according to a Wikipedia entry. The average height for a wheelchair user must be the same measurement as well.
But if you are searching for the average height of a person while they are seated in a wheelchair, then you are looking for the eye level of a wheelchair user. The eye level of a wheelchair user can range from 43 to 51 inches from the floor to their eye level.

Thank you so much for sharing your experience. It is wonderful to hear how the chair has impacted your lives in such positive ways. Your mother sounds like an amazing person. Our team here wishes you and your mother the brightest of days ahead!