N.B.
This will come up a zillion times, so you may as well deal with it now. We all have to teach our kids, "Our house, our rules." "They said WHAT? They did WHAT? They bought WHAT? They watch WHAT on tv? We don't do that here." This will cover anything they see, hear, or want to do at friend's houses. Maybe you could talk to the other mom about what would be going on "if" your daughter were to spend the night. You can discuss what you expect, and if she doesn't agree, then you have an out. Please think about the other little girl. She will need all the friends she can find, so your house may be a refuge for her. (Drinking usually means fighting.) At least she can see what a stable, loving home is like. So many kids think the most awful things are "normal" family life.