Final answer:
The heuristic at play was the representativeness heuristic, where the observer assumed professionals' roles based on stereotypical norms.
Step-by-step explanation:
The type of heuristic used during the initial reaction to the two individuals is the representativeness heuristic. This is a cognitive shortcut where individuals categorize someone based on the extent to which that person represents the typical member of a category. In this case, the assumption made was that the man was a physician and the woman was a nurse, owing to stereotypes associated with those professions, rather than judging them based on individual characteristics or statistical reality.