Yeah, definitely a tricky one. Even trickier in another couple of years, as your control lessens (and your daughter's perceptiveness increases). I find that a a multi-faceted approach is the way to go. First, I do think it's OK to have discussions about behaviors and habits, especially in a way that doesn't directly make the new friend the villain. It's important that your expectations for your kids be repeated. Second, while I wouldn't "ban" the new friend (unless there was a dangerousness issue or the like) you still do have the ability to affect schedule. Which activities do you choose for family? Where do you go? Are you encouraging your daughter to remian active in multiple areas? Reinforcing and modeling the behaviors you like and providing her lots of opportunities to meet lots of different kids should in the end help her to find a peer group that you're pretty comfortable with.