> My concern though was that her allergy might be so bad that just having a little tuna juice on her plate, or for her other sushi to be cut on the same chopping board or with the same knife as the sushi could kill her.

I think that would be their concern, too. If they gave you a meal and didn't add the tuna, how can they ensure that every station, utensil, and dish hasn't touched something contaminated by tuna? They probably can't.