It might be worth being careful. I got food poisoning after a trip to Singapore, and based on when the gastro started, I *think* it was from the plane food (BA J) rather than anything I ate on the ground in Singapore. I can't remember what type of food poisoning it was, but it wasn't listeriosis. I have had that type of food poisoning before (from under-cooked gyoza in a sushi place) and was from chicken/pork being under-cooked.
That being said, I had two BNE-LAX-DEN-LAX-BNE trips while in the earlier stages of my pregnancy in QF Y, and had no issues. And no issues with the food on flight in BA J to Singapore.
I'd be careful about taking the chicken or fish option in Y, and if your fiance wishes, perhaps get the Vege option. If she's unsure, best to check with her GP/OB/medical carer for advice. After all, these are just my opinions and experiences, and I'm not a medical expert.