Talk to your child alone first and point out that you are aware of her behaviour when she is with her peers. Clarify your rules regarding swearing and warn her the next time you hear them swearing you will interrupt.
Don't be apprehensive about parenting your child. Parenting is about respect and understanding, and not being about a friend. Through your earned respect, you will enjoy a peer-like relationship when they are adults.
---- Sent from my 3 mobile Nokia E71