I think what needs to be determined firstly is the difference between a persons survival instinct and a phobia. A phobia is defined as a type of disorder in which “the affected individual displays a marked and enduring fear” (DSM IV: Specific Phobia) of something. While a basic survival instinct is defined by physiological changes in the body designed to ensure ones survival when the situation calls for cautiousness. The primary dissimilarity here is in how the feeling endures. Instinct tends to only become evident when a risky or dangerous situation is close at hand, while in the case of a phobia a person could be fearful of just the mere thought of a situation despite this thought obviously not placing them in any danger. With this considered it is clear why most would choose to back away from a tall ledge while permitting themselves to be at a near enough distance to fulfill their natural curiosity and perhaps get something of an adrenaline rush from the experience. In this regard I would conclude that humans are not acrophobic by nature but are indeed fearful of heights when personally presented with them. Concerning your friend, my suspicion is that she has either become so accustomed to performing the activities you describe that her survival instincts in this context have dulled somewhat, or she’s simply putting on a show of confidence and is still apprehensive of the possibility of falling but disregards this in favour of excitement.