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The police officer must have a reasonable suspicion in order to follow somebody's car. If he did, in fact, clock her for speeding, that constitutes reasonable suspicion but his response to that reasonable suspicion should be to pull the car over as soon as practicable. The further he follows the car without any action, the more inappropriate it becomes. However, the police officer is under no obligation to issue a speeding ticket or a written warning. A verbal warning that he issued is a proper response.
So, it depends on how far he followed your daughter. He may say that he decided that the speeding incident was no big danger to traffic in that case, so he decided to safely pull the car over in a residential area. Chances are, he will get away with such a response. unless it is the same police officer that is doing this to her for the second time.
If smth like this happens again, your daughter needs to write down the badge number and consider following up at the police station to express her concerns about possible harassment. At this point, however, we don't have enough evidence to say that his behavior was inappropriate.