In the case of this 'one friend', she was not handcuffed to a job because of equity considerations. Therefore, no problem for her to quit.
Kind of works against your original point, not for it.
Not sure why this would be surprising -- this situation is very common across startups. I am a co-founder also, and I have equity vesting agreements, understandably, as I want retention; I don't want employees constantly leaving with the next wind that blows. At the same time, I'd hate to keep a miserable employee who isn't fully productive. I'd like to think I set a good enough workplace culture that there wouldn't be harassment/bullying, but who knows...
What is more wicked, and beyond me as a business owner, is US tax treatment on illiquid equity. That really needs to change.