It's rude to keep tabs on someone's eating habits.
Rebecca is with her friend at a restaurant and her friend ordered something unhealthy.
Rebecca wants to have a conversation with her friend as they eat.
Rebecca talks about the weather with her friend and other such day to day things.
Rebecca passively notes aloud that every time they go out to eat her friend picks the most unhealthy foods.
Rebecca's friend enjoys the conversation and meal with Rebecca.
Rebecca's friend is insulted by Rebecca's observation and becomes self concious about her choices around Rebecca.