D.W.
Our son is also a follower by nature.
He's been in day care almost 2 years and will be 4 in June. There are a couple of trouble makers in his class - kids we'd never associate with normally. There's also a little boy across the street who, though being a sweet kid, has no direction from his parents and says/does things we don't accept in our house.
So when he does something he's learned at school, we remind him that we are his parents, and these are our rules. We are very firm and don't bend on most of them, and he knows there are consequences to certain things - for example, calling other kids "Poopyhead". Not something I want my kids being known for. The more he does it, the more consequences come his direction.
As for the kids at school, we regularly talk about the kids in his class, ask who he plays with and comment really positively about certain kids. We've really tried to get to know the teachers and the other kids in the class and have an open dialogue with the teachers in the event there's something that we need to address with them.
Without coming out and calling her friend a bad kid, I think you can talk about behaviors that are good and bad. We're really making a concerted effort to praise and reinforce good behaviors. When a gentleman held the door open for us to walk through at the mall last night, we commented on how nice his manners are and how we'd love for him to do the same things.
Hopefully your daughter will start recognizing that this friend isn't a good example and that the consequences of following her aren't too much fun either.
Good luck - I keep hearing 4 is a little better than 3.