From the Mayo Clinic

A shellfish allergy causes an allergic reaction when you eat shellfish. Sometimes, a shellfish allergy is only to certain kinds of shellfish, or you may have an allergy to all shellfish. Shellfish include marine animals with shells, such as shrimp, crab, and lobster, as well as octopus and squid.

I've known quite a few people with shellfish allergies, none of them would touch any shellfish, they did not feel it was limited to just one.

The safe way to find out is to get tested by an allergiest or whatever. There's no way to tell by sneaking crap or lobster into her dinner one day to see if she gets a free ride ambulance ride or not.