Ethically neither one of the people in the car should be arrested. However, possession of illegal substances is against the law so the driver rightfully should be arrested. Although I do not agree with the reasoning behind the police man's actions, the car did in fact contain marijuana, therefore this fact should not be dismissed, ethically.
The article does not say anything about consent to a search from the suspect, so I believe the evidence would not hold up in a court of law. Personally I think it should though. Without the consent of the suspect or a search warrant, the police officer should not have opened the door and taken the bag. The article says the police officer only saw the edges of a baggie. The baggie could have contained a sandwich. Yes, this would be very unlikely but it is possible.
The Terry vs. Ohio case is an example of why the police officer had no right to stop and search the vehicle based on reasonable suspicion.
A black man and a white woman should not qualify as reasonable suspicion anyhow. In this case two men were stopped and frisked because the police officer claimed they "did not look right." The supreme court ruled that searches may not be conduct on reasonable suspicion alone, and the evidence found cannot be used against the offender in a court of law.