Without knowing their specific allergen, no one can answer this. The miracle of modern society is that there are alternatives to virtually everything, so its enough to say that a person is allergic to "X". Unfortunately, that doesn't tell us what other things they may (or may not be) allergic to, bucause the've identified the food containing the allergen, not the allergen itself.
Example. Many people are allergic to "shellfish". A very significant number of those are allergic not to shellfish, but rather the products of dinoflagelates in the local waters. These people tend to be very sensitive to red tides - but can often eat shellfish from waters where the dinoflagelate levels are much lower. Others are allergic to byvalves - its the tropomyosin (usually), but think they are allergic to shellfish too, because shellfish are so often prepared in the same pans, withthe same oil, as the bivalves are fried in. and as these same people are unlikely to eat raw oysters, they never find out. Or, they find the oysters have accumulated thesame products of the dinoflagelates that causes their shrimp and crab alergy.
Knowing that they are allergic to "shellfish" helps them make dietary choices to avoid risk. Its enough for them to live their lives. But its not enough to assist others in making predictions about other foods which might trigger a reaction.
Hope that helps, even though it doesn't answer.
