An ambulance is cheaper than being mugged or s*xually assaulted.
True, though I know exactly what's to blame in this situation: the whole "making a scene is absolutely the worst
thing you could do" idea we all got pounded into us.
Perhaps Kelly would have called an ambulance if she hadn't run into the park employee, perhaps she wouldn't have. Perhaps the pain didn't let her think clearly - no offence meant to Kelly, I know thinking under pressure is extremely hard and I envy everyone who can do that from the bottom of my heart.
Call me evil-minded, but I wouldn't be surprised if, between the time Stacy left Kelly behind and the time she caught up with The Cool Girls, Stacy realized she could use Kelly's absence to make herself look better.
"Oh, look, the experienced runner got tired and gave up already while poor, feeble little me has enough force of will to see the run through. See, she wasn't really hanging back because of me! I'm so much better than her already!"
I'm not saying
that's what she thought, but...it would explain the lie.
OP said Stacy was sort sort of distant with Kelly after they started running with The Cool Girls - isn't it possible that Stacy was jealous of her? That she started seeing Kelly as competition for The Cool Girls' attention?
The three of them already have a genuine love for running in common, they're all more experienced than her...
I know, I know: speculation and we haven't got a shred of evidence from OP's story. It's just a thought.
However, it has been said it's probably not the first time Stacy brushed Kelly off for her own, selfish reasons. I'd also look back at her actions whenever she had been jealous of someone else.
Again, just a thought.