Are humans acrophobic in nature?

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.

For me it depends on how precarious the situation is. For instance. I have no fear standing on a sturdy building or cliff, looking down 20 stories or more. However ask me to climb a ladder higher than 6 feet in height and I become a frozen statue. I don’t trust ladders or anyone carrying me or the nature of a trampoline. I probably wouldn’t trust a cliff either if someone is standing near me. I would be afraid they may bump into me. I can climb a tree and go on thrill rides. Anything I feel sturdy on. People and unpredictable things make me a bit nervous.

No. I was never bothered by heights until sometime in my late thirties/early fourties. I found myself paralyzed on a bridge (walking) that I had walked across many times. I had to get on my all fours and crawl back to the land. I have been like this ever since—just cannot handle heights at all; I will simply faint if forced to be aware of being high.

I don’t see the need for the long answers.

Yes, most animals for whom heights are deadly have an instinct that tells them so.

No, it is a specific phobia that only some people have.

No. I love heights!

Was high as a kite, literally, in a hot air balloon last year, freakin’ LOVED it!

I have jumped off bridges above wild rivers, lain on my belly on cliffs with 1000’s of foot drop offs in Bryce & Zion state parks in Utah, perched on the roofs ledge of my 10th floor apartment in Taiwan last year, every night, for the view of Taipei city.

Have ridden horse for years.

The only thing I fear are ticks…aaaah, had one on my shoulder once, horrible, evil things! lol

I think we are in nature yes, but I think people like @Coloma have learnt to overcome the fear and positively embrace experience, finding the positives in life… which is commendable.

To those answering yes, there is a substantial difference between a fear and phobia. A phobia is an extremely restricting, irrational and enduring fear, while a fear alone is simply something that, when presented with it, can make you anxious or panicked. If humans were naturally acrophobic no tall building would exist as the majority of our species would refuse to enter one.
Yes, humans are fearful of heights, some more than others, but we are not acrophobic by nature.