Although some here have said that they would tell, and in some cases, have told, their running partner to leave them, despite a possible injury, how many would actually do that 5 miles out on a trail that isn't safe to be on alone? Besides the fact that you'd be left by yourself, you would also be letting your running partner go on alone.
How many of you would actually leave a running partner behind in that situation? I'd back off trying to actively help my partner, but stay close by. My friend could tell me to ditch him/her all they wanted, but given the safety risk to both of us in that situation, I couldn't be made to do that.
Stacey's behavior when she left Kelly was reprehensible, even without considering the way she acted afterward, which eliminated all chances that she could blame an astonishing lack of safety awareness.