I would ask her how she wants to present herself. Swearing is crass and often makes the person sound less intelligent instead of more mature. Her friends may think it's "really cool" and "grown up" but it just sounds vulgar. Just like what to wear, what you SAY matters. My sks grew up hearing people swear (not in our home, generally) and managed to filter themselves. What I would be more concerned about is if this indicates a shift in attitude and behavior that you do not want your DD to be a part of. If they are still nice, perhaps invite them to your home and hear what they say. Then ask them to rein it in. When SS first returned from college, he'd drop curses more often. We'd caution him ("Please remember you are home now. You're dropping a lot of f-bombs and we don't need to hear that.") and he'd usually straighten up.
You may also ask her about her friends. Is she surrounding herself with the right people?
I would keep an eye on this because if she returns every time with a curse, then perhaps you need to not give her so much time in their home. See above about inviting them to yours instead. Sometimes rather than banning a friend, we had them come here, where we could keep an eye on things. One of the neighbor girls (about 13 when SD was 11) showed SD and her little friends how to get on a chat line. Had SD been to her house, we might never have known. We talked to the girls, talked to SD and moved past it, but inviting the girl to our home confirmed what we suspected and SD was no longer allowed over there.