I'm a huge underreacter. Particularly when it comes to my own injuries and illnesses, but also sometimes when it comes to other people's. I could see Stacy's original actions as not being reprehensible. Not morally awesome, but morally neutral, or mistaken. I could see me thinking 'oh, she twisted her ankle, no big deal'. It wouldn't be the right call, but it could be unmalicious and her thinking 'well, if I had done that I'd want my friend to go on, I don't like having people around me when I'm hurt...' To someone like me 'I'm in a lot of pain' does not necessarily mean 'Please stay with me.'
But, Stacy's actions afterwards? No, just no. If it were me who had made that mistake, I would be falling all over myself apologizing. I sure would not be minimizing it!