Hi Colin,
Had something similar to this myself and like you, zoned on on what appeared to be the most obvious symptom, this limitation on External rotation. Try to take a step back and look at the bigger picture, for example does the patient have "Lower Cross Symptom", You spotted the weak Glutes really well, but does she also have a weak Core and over-active hip flexors, if so try doing some "Illiopsoas Release" BUT DON'T GIVE OUT THE ILLIOPSOAS STRETCH, Trust me it will only lead to disaster!!
I too would look at her gait/run (on a threadmill if possible) so you will be able to spot a restriction if there is one. Are her QL's taking over from another muscle?..., I would take full advantage of the fact that she appears to be pretty low irritability and look closely at some of these things.
Just try not to focus too much on the lack of external rotation, as it may only be part of the picture!

Do let us know if any of this helps"!
Kind Regards,
Sarah