My daughter's bff for the last 3 years is not someone my mother would have let me play with. She has TERRIBLE hygeine habits (really, it would make your head roll) and her dad is a pot head. They are really really different from us.
We have strong values and are very concerned with teaching our children good financial habits. At three years old, my son knows that if he wants anything from a store, he has to earn the money and he does so. We practice saying "no" to buying them stuff so that they will not be irresponsible with money when they grow up. BFF's parents buy her an ice cream every time they pass the ice cream machine at the gym. If we are all together, I still live by our values and tell my dd no, we're not spending money like that. So my dd knows that just because they do, doesn't mean we do, too.
BFF has been watching horror movies with her dad. We don't own a television. BFF watches shows meant for teenagers and I overheard a conversation when they were 5 years old where she told my dd that you need to dress "sexy" to get the attention of boys you like. Of course I jumped in and asked her what that meant to her, and explained our values.
I adore her mother, but we're different. They are really good people, but we have different values. The little girl is amazing in so many ways, but there is no way my mom would have allowed the friendship. I allow it. She's a great friend to my daughter and brings other great values to our table.
I don't discourage the friendship, but I do talk openly about our values and expectations. We talk about how we are different and why. By example, we show our kids that you can love people different from you and still be YOU. Some day your kids will make their own decisions about friendships and will be subject to peer pressure. My thought is, why not show them how to retain their own identity and still be comfortable with those who think differently. I'm hoping that those lessons and experiences will light her path when she is older and not so much in our sight any more.
They have us to guide them. My daughter knows the difference between right and wrong and she'll tell her BFF that she's not going along with certain things. Then she tells me about them. I'm over the moon about that.
Good luck to you. Sorry for the book.