but she has a seafood allergy (not just eating it, but being near it)
Call the airline's reservations number and advise them of the allergy once booked. Air Canada, and other airlines, have procedures in place to deal with these situations. Calling customer service may, or may not, be as effective.
With peanuts other passengers are requested not to open peanut packages or consume brought on food that include them.
Edited to add: Make sure she has her 'epi-pen' when she travels. They are not subject to the liquids restrictions.