Earlier quoted context omitted.
Approaching a young child who has been left alone in a car is foolish in today's paranoic society. Also, the returning parent is unlikely to be grateful to the comments of "I was just making sure your left alone child was not dying".
The person was near enough to spot the child and take a photograph, so they could have remained at that distance and call the police if they feel it's necessary, not least to advise the returning parent/guardian that the police were called. In that case the police would probably have arrived and written a ticket or something. Although this would have involved a delay and frustration for the temporarily unattentive pa…
No "someone else", but the very same woman: leaving the car, returning back to the car. The same woman, not someone else. Unless the good samaritan has deceptively shown just the last part and told the police he (or she) saw a different person leaving the car with the child inside.
The issue here is not car theft, driving license not matching the car owner, or whatever. It was instead a good samaritan evaluating as criminal/dangerous the fact that a mom left his son alone in the car for few minutes. And decided (when she came back) to call the police to...? Check if the child was fine? Whatever.
It takes few minutes to check if the car was stolen and/or the owner knows that someone else is driving it.