As a parent you can offer advice, place restrictions, and continue to be a good listener to your son. Your opinion of the girl is not his. Discuss with him what bothers you about her but expect him to be defensive. Keep listening, provide a place for them to be in your home where you can chaperone, and offer food and entertainment. If he doesn't buy into it, speak to her mother and get to know her better. Cooperation is better than condemnation.
Wayne Yankus, MD, FAAP
expert panelist: pediatrics
Once he starts dating her, you don't have much of a choice. To get your son to stop dating a really bad girl, you should have started when your son was 2 and 3 years old. You show him what really good girls are by associating with really good families. Bank tellers know counterfeit money because they deal with the real thing all the time. Your son made the wrong decision when you allowed him to be familiar enough with this girl to even want to date her. Your son should be so used to obeying and loving his parents he wouldn't do anything against their wishes. At this point you are going to have to involve yourself as much as you can with this girl so she eventually changes her ways, or tires of this family that is 'too good' for her.