A quick internet search shows but alergys effect 1 in 200 UK adults, and that most get mild discomfort rather than life threatening reactions. I don’t think it’s reasonable to take them off the menu entirely for that level of risk. I think that if someone makes the airline aware ahead of a flight and presents medical proof of how risky they are then perhaps they shouldn’t stock them on that flight, but this women waited until she boarded and then expected the flight crew to make the decision.