Parenting a teenager can be very stressful. Teenagers are notorious for doing the opposite of what their parents tell them, especially when it comes to choosing their friends.
Avoid telling your teen that they are forbidden from hanging out with their best friend. A more effective approach would be to sit down with your teen and discuss the issue without making any blanket statements about their friends being "bad influences." They are more likely to tune out adults that use negative labels to describe their friends.
This issue goes deeper than just keeping your teen away from their friend. Take this opportunity to discuss the underlying issues of why they are choosing to hang out with them. These issues include decision making, healthy relationships, and boundaries. If your teen doesn't know the difference between a healty and an unhealthy relationship, spend some time talking about what it means to be a good friend. A good friend is a good listener, willing to be there for you when you need it, and doesn't get you into situations that may get you into trouble. Lastly, they don't use or manipulate you, and are willing to stick up for you in tough situations. Can your teen say that their best friend is doing those things? Ask them to think about the friends they have and whether they fit the description of a good friend.
During this conversation also discuss their decision making skills. Ask them what goes through their head when make a choice. Do they think about the long-term consequences or are they simply thinking about that moment? It can be difficult to get a teenager to look at their future, but with some careful discussion you may be able to at least get them to think twice before they make a bad decision because someone is pressuring them to.
Sometimes kids are more apt to listen to advice if it doesn't come directly from their mom or dad. Is there a trusted relative that can talk to your teen that may be able to influence them to broaden their group of friends?
Be honest and open with your teen about how you feel about their best friend but give concrete reasons why you don't feel like they are a good influence. Then it's up to them to make the choice.
Best friends are a really tricky thing, as I know many children are as close as siblings (or closer!) with their buddies. Growing up I was fortunate to have 3 best friends who were all of great character, and we all ended up mutually benefiting each others development (still best friends with them today).
I feel like any direct confrontation will backfire (how would you feel if you parents told you not to see your best friend?). Instead, I would suggest trying to encourage your son in new activities or interests (hobbies, clubs, jobs, etc.). Not only will your son hopefully gain new friends with similar interests, but as your son changes so will the dynamics of his friendship with a bad influence.
Well, teenagers are a breed of their own. I can remember hanging out with some really bad hats but my parents never "made" me change friends. However, they really needed to intervene at times. Sometimes teenagers are exploring and trying to find out where they fit in. And sometimes this means sacrificing their own morals for the sake of having friends.
No child wants to hear "You can't hang out with.." especially a teenager. The bad influence has somehow convinced your child that he/she is his/her "best friend". If course, they need a good friend to use as a crutch if all their wicked ways come crashing down. Unfortunately, your child has to learn this on his/her own and there is no easy way to learn it.
However, I do feel that the parent has the upper hand. You don't want your child resenting you. So one thing to do is approach it with caution. Remember you ultimately have the power to approve or reject outings with this bad hat. The "bad hat" can't outsmart you and you have to let this friend know this is a subtle way.
So I would very cleverly keep my child away from the influence. As parents, we have the control ultimately but I would expose my child to other kids/activities. Be very pro active in your child's life. Try to come up with activities that would steer the child away from certain people. Eventually, your child will see they can meet other people who have a better interest in them.