What is your opinion?

Ok, I have a conundrum. What is the proper way to handle you pre-teen child's friends? I have my own opinions about this topic but I really would like some feed back good and bad about your life experiences, either your first hand ones as a pre-teen or as a parent of a pre-teen. If the friend's manners are lacking and you are hurt by the lack of kindness, what do you do? If the friend is a bad influence (behavior wise) what do you do? Just wanted to have your perspectives on this. Thank You. Dawn