Can hanging upside down make you grow taller? My friend claims she has noticed a difference

Myth. Any temporary increase in length will be lost soon after you stand up and gravity returns things to their normal position.There is a strong seduction to anything you want to believe in. We know with sugar pill tests that 30% of people who "believe " in a fake treatment will give it glowing reports.
No. Absolute nonsense. If both of you are teenagers, you're still growing. Any increase in height she is attributing to hanging upside down is due to normal growth at her age.