I think the most reasonable thing to do would be to not fire the Muslim flight attendant, but yes move that flight attendant to flights where alcohol is not served. Or have the flight attendant politely explain to passengers that she does not serve alcohol - either because of a religious objection (which I don't think really needs to be publicized) or a phony excuse, that only one flight attendant can serve booze and then to say that she will tell that flight attendant what their order is and she will serve it. Kinda convoluted but that is the best course of action... regardless of the religion - if the flight attendant didn't want to serve alcohol, let's say because they had been abused at the hands of an alcoholic as a child, then reasonable accomodations should be made.
No need to fire perfectly good workers for things like this!
I mean, it seems kinda stupid that serving alcohol would be against her religion, but telling another person to serve alcohol wouldn’t be.
