Could there be an associated tendinopathy in the area that is causing increased pressure / irritation to the bursa? Other than the bursa, can you find anything related to gluts or hams (muscle belly or tendons)? If so, would you be able to treat as a tendinopathy without irritating the bursa (ie. eccentric exercise, local soft tissue techniques/frictioning to the tendons)?

If she does sit a lot, I would probably try to focus on finding a way that she can sit without continually irritating the bursa. A few thoughts...
1. soft chair / cushion?
2. the "doughnut pad" idea comes to mind too but can't think how this could be done for the ischial tuberosity without causing discomfort to the perineum?
3. remember those chairs that were marketed years ago to decrease back stress (but then people found out they are really hard to get out of without falling over and hurting your back...haha) that had a pad where the shins would rest against and the actual seat was on an angle? The pressure may still be on the ischial tuberosities (if in good sitting posture) but at least some of the body weight would be put on the legs and thus taking some of the pressure off the bursa???
4. How is she on a ball? Again she would still have pressure on the ischial tuberosities but A)it would be a wee bit softer than a chair and B) the direction of the force may be slightly different just due to the shape of the ball vs. a flat chair.