Final answer:
A guest with a shellstock allergy should avoid all shellfish and be cautious of cross-contamination or dishes that may contain shellfish extracts.
Step-by-step explanation:
A guest with a shellstock allergy should avoid foods such as shellfish, which includes creatures like shrimp, crabs, lobster, clams, oysters, mussels, and scallops. Additionally, it is important to be aware of cross-contamination which can occur when shellfish is prepared or processed in the vicinity of other foods. One should also be cautious of dishes and sauces that may contain shellfish extracts, such as certain Asian dishes or seafood stocks used in soups and stews.