THORACIC MOBILITY. 9/10 times it's either this and/or poor posture (shoulders hunched forward too). If the T-spine is tight and hunched a bit, you're not going to be able to get the elbows up well enough even if the shoulders are more than flexible.
Foam roll it... then do some hip extensions getting her used to the arched back. Some stretches would probably help as well such as BAND dislocates (better than PVC).
How's her kipping pullup? Posture?