Just to recalibrate the discussion to the facts - heaven forbid
BA customer facing staff are not allowed to wear any visible jewellry (except, I believe the usual wedding ring and pair of studs). THe fact that the necklace was a cross was irrelevant except in the mind of the nutty bird who is complaining. Her reason for complaining is that she sees the cross as a necessary part of her religion and if a turban is visible, then why should her cross not be. BA say that you can hardly hide a turban (or Sikh bangles) but it would be very easy for the small cross to be worn beneath the blouse and hence comply.
Seems perfectly reasonable to me.