What gets me, is how Stacy basically told the Cool Kids "Oh, Kelly's had enough running, and is heading back to the car park."
This is so far from the truth, that it's staggering.
The most charitable explanation I can think of, is that Stacy had a moment of supreme misunderstanding, and somehow genuinely believed that Kelly had only suffered a very minor injury (not worth mentioning, really) and was leaving because she didn't feel like running anymore.
However, even if that was the case, Stacy should have been apologising PROFUSELY to Kelly once she'd learnt that Kelly had broken her ankle. As Yokozbornak stated above, Stacy's behaviour after the fact is appalling. That's why I think that Kelly should sit down and have it out with her.