Countless proverbs warn of the strong influence your daughter's friends can have on her, such as "Show me your friends and I show you who you are." While you do not want to belittle your daughter's friends, you do want to have your eyes open and notice their appearance and the way they act, especially how they conduct themselves when they think you cannot see them.
As a parent you have one great weapon: your instinct. When a friend of your daughter gives you an uneasy feeling, examine the reason. Is this girl dressed strangely all the time? Does she wear outlandish makeup every day? From age twelve on, some girls consider adults the enemy and want to get back at them by exposing body parts, wearing underwear as outerwear, and getting lots of tattoos and body piercings. Also read what it says on their T-shirts or what they doodle on their notebooks.
Be aware that a Lord of the Flies effect has been observed among preadolescents and adolescents in which the lowest common denominator always prevails among a group of friends. Therefore, if your girl associates for a long time with girls who act much more mature or are engaged in destructive or unhealthy practices, she will be tempted to take part in them as well.
Talk to your daughter about what you have observed about her friend. Perhaps she is feeling sorry for this particular girl because other kids make fun of her. Compliment your girl on her kindness and see what you can do to help this "weird" dresser fit in better. Or perhaps this is just an unconventional girl who is very creative and artistic. Help her shine in some way in front of the group. Whatever the situation may be, do not let your girl abandon her other friends and spend all her time with girls who give you a bad feeling.
You do not want your daughter to gravitate only toward girls with problematic backgrounds that she cannot solve at this stage of her life. Instead, encourage her to make friends everywhere—at school, at athletic practices, in the neighborhood, and at her place of worship. Having friends from all social spectrums and ethnicities can be a big plus for your daughter. The more she widens her circle, the more she can learn from all kinds of talented girls.