hi sports_Rx,
have you cleared for stress fracture? is it painful on ultrasound or tuning fork tests (good tests to do without having to do a full bone uptake scan)?
have you taped for offloading the peroneals and seen if that made a difference to pain and function.
have you checked biomechanics in the runner's shoes (are they old/worn/collapsed laterally?). is she a lateral heel strike, pronator/supinator runner? - put onto a treadmill and watch running... if you have time, get her to run until the pain starts and see if her gait is fatiguing or changing. perhaps some advice on footwear or supports is needed if she is going to increase running significantly.
if all that is clear then it does sound like she has increased her distance in training too fast - overuse issues. 10% rule should apply for incremental training increases. so a rest, some cross training, stretch and strength would seem like the way to go.
good luck.
msk101