Sounds very specific the the amount or extension in relation to the external rotation. As dancers walk in more external rotation in normal gait I suspect she does the same. The frgo post would have her in more relative extension with the external rotation than the FABER test.

Initially reading I was thinking about the other leg during gait so it is great you mention it. e.g. A lack or ability to internally rotate from mid to late stance will mean that the contralateral hemipelvis will remain slightly posterior (in reference to the horizontal plane) during that sides swing phase. This will cause a heal strike in a postion of excessive external rotation (not really in the hip but due to the direction the pelvis is facing) and therefore excessive pronation through stance (an unstable stance therefore) which will lead to a less that ideal heal strike again...etc...etc. I suppose what I am saying as this can get rather confusing, is that a problem with the rotation of the leg on one side during stance phase can cause problems on the other. As your observation suggests this then I would begin by treating the restriction and normalising the movement on that leg and see what affect that has.

Good luck and keep us in the picture