My reply may not be of great use but would like to do some brainstorming at the very least...
Case1:
I'm making an assumption that she has pain and/or weakness with hip flexion....what are the results of the rest (ROM, muscle testing...)?
When she gets the pain during stepping, is it anterior hip pain? Is it any worse (or better for that matter) during running or quick stop / start in soccer?
Does she get any clicking / popping in the hip?
Case 2:
Could it be that she just had her leg in a sustained position for too long while in bed that day and just flared everything up? (i.e. either sustained extension if lying on her back / sustained valgus position if she as on her side)
Probably not that simple but just a thought.....
Case 3:
Has the burning gotten any worse in that last 6 months or is the same discomfort she's had for 2 years? Is it a fairly diffuse area across the low back or can it be localized? Can you reproduce the burning with any testing? What are the results from your assessment?
Is the burning a "nervey" / "zingy" kind of burning? Or is it more like a muscle fatigue kind of burning? (I know that may be tough since it's not your pain...)