I'll jump in.
Had Kelly been clear, then Stacy would be clearly in the wrong. On the other hand, if Stacy had said to the friends "oh, Kelly twisted her ankle a bit and went back" she wouldn't have looked bad. So the burning question for me is why she didn't say that. Why didn't she tell the friends that Kelly had gone back? Because that would have been the normal response. And it wouldn't have made Stacy look bad at all (at least not to me, if someone says they are heading back because they twisted an ankle I would be fine with that). Stacy's reaction even before she knew Kelly was more seriously injured is pretty unflattering.
I can speculate as to what her thoughts were. I can suppose she wanted to get in with the cool girls, or didn't want to interrupt her workout, or just wasn't thinking because she was caught up in runner's high, but in the end, the reality is that she either didn't stop long enough to ascertain if her friend was hurt enough to need her help, or she chose to leave anyway. Neither would have me keeping her as a friend or workout partner.