People who are friends with celebrities are surely used to getting burned when out in public with their famous friends. Strangers, of course, want picture swith the celeb and not the plebeian friend. The friend surely becomes familiar with this process of getting the shaft. Unless that friend is also famous, then they'll likely be confused or bemused as to why they're not wanted in the photo as well.
Twitter user @HonoredSpirit shared such an incident as documented by his friend, who saw Matthew Broderick and wanted to take a photo of the star and his daughter.