If I came to u, said I was allergic to fish and you served me a dish with fish in It anyway.....if I survived, you wouldnt.
The first time she tried it was when I took it to a potluck and I had no idea of her "allergy", she emailed me the following day telling me how good it was. Didn't think anything more of it until at least two more times later when Mrs E mentioned how funny it was that someone so allergic to fish had no problem with a dish loaded with fish sauce, at which point it seemed easier to just say nothing.
I agree that if I had deliberately given fish-laden food to someone with an allergy and not said anything it would be a dick move, but that wasn't the case. At this point it's probably best left alone.