I think FA's have many things to take care of.
In a training classroom, to the question "If a passenger has told you that he/she has a severe allergy to shellfish, would you serve a spinach linguini main with a side of crab salad ?" they would probably reply "hell, no!".

I bet similar situations are not covered by training and given the busy time of the day that FA simply did not connect the dots.
Primary responsibility rests with the passenger: the more serious your condition his, the more careful your scrutiny should be.