Juanita sees a crowd of people gathered around a man lying on the ground. she thinks about whether to call an ambulance, but then reasons that somebody else has probably already done so. this is an example of

 Diffusion of responsibility

Diffusion of responsibility it's when a person is less likely to take responsibility for an action when others are around. The person assumes that others should take responsibility for the action or have already done so. Just like Juanita did, she just assumed someone called the ambulance, so she didn't do it herself.