“The hotel agreed to stop giving her the Sunday shifts for a lengthy period between 2009 and 2014. However, this ended in 2015 and they eventually began “demanding” that Pierre start working again.”
And that’s where they lost their case.
If someone tells you that, for religious reasons they will not work on Sunday, (or Saturday or whatever their rest/worship day is) and you arrange to accommodate them you must continue to do so.
You have accepted their terms.
Had an employee who wore a beard for religious reasons, when the new owner came in he was told he would have to shave.
I checked all the rules, federal and state and told them that they would just have to live with his wearing a beard.