Natalie is correct in saying that prone hip extensions are a way of working the glutes without getting the knees involved. SI joint may be another story if there is instability causing the pain.
What really struck me, though, was the question what made you single out the gluteus maximus as the muscle that needs work. Maybe it does, maybe it does not. Has she seen a doctor or physical therapist so that you know what exactly is going on?
And as a yoga teacher, I assume that she also practices yoga in addition to walking and swimming. The root cause of her problems can come from any one of those activities.