divebuddy - "//The reasons why the tasks are not performed are of no concern, or relevance in terms of enforcing an employment contract.//
So why had the company previously allowed her to carry on with her job and be excused from serving alcohol.
That "arrangement" has broken down (probably because her fellow stewardess's got fed up with it).
By being reasonable they have given her legal team (and it is a team) a big stick to beat them with."
I suspect your analysis is correct here - the airline have attempted to accommodate this lady's religious convictions, but having been unable so to do, they have reverted to their right to enforce her contract regulations.
I don't think they will have any trouble in court - accommodating religious or other cultural aspects of an employee's working life are the courtesies of a caring company, but they are not obliged to provide them.
However, the employee is obliged to carry out the tasks in her contract, when required, and it is she who is failing in the legal sense, so I suspect the court will side with the airline here.