I haven't read the entire thread, so I'm not sure if this has been said or not.
I don't think the issue is with what happened on the trail. Yes, Kelly was injured, but did not ask Stacy to stay with her, nor did she mention she was injured. That's been established.
However, if I were Stacy and I found out later that Kelly was indeed injured, my response wouldn't be to be ANGRY at her. I'd be apologetic for leaving her, and maybe I'd ask, "Why didn't you say something? I wouldn't have gone on if I knew you were hurt!"
I wouldn't be trying to blow the entire thing off, I wouldn't be calling Kelly's husband to try and talk some sense into her, and I wouldn't be constantly defensive and trying to shift blame around to assuage my own guilty conscience, because deep down, I'd feel like a jerk for having left my injured friend. Of course, these are all assumptions. I am not Stacy. I wasn't in the situation myself and I cannot speak for anyone else.
So, in short, I think the issue isn't so much what happened on the running path, so much as the way Stacy handled it afterward.