Definitely have your daughter in extracurricular activities that do not involve her friend. Is your girl in anything outside school, such as Girl Scouts, sports, dance, art classes, book club at the library --anything? If she is, but the other girl and she are always doing these things together, it's time to let your daughter find an activity that interests HER (not based on what her friend's interests are!) and get her into it, preferably more than once a week if her school schedule allows.
And if the girls are in lots of classes together at school -- you can't do much about that for this school year, but around here, principals do try to honor it if a parent requests in writing that their child not be in a class with another child the next year. Usually you do not need to give a reason. Find out the policy in your school district. Schools are not required to honor such requests but often will. Your daughter does not need to know anything about it.
I'm not saying cut them off cold turkey; that would make the friend "forbidden fruit" and therefore even more interesting to your daughter. But nudge your child toward constructive activities that do not involve this girl. Make sure your daughter has the most input and final say on the activities chosen, so it's really something that interests her. Meanwhile, ignore the mom and maybe even cool your own contact with her, confining it to planning the (decreasing) get-togethers. And as someone else said, limit the time frame the girls are together. I would add: Be sure they are doing a specific activity when together -- watching a movie, doing a craft they both like, going to the library together (not the mall with all those clothes!) etc. -- rather than sitting around comparing clothes and talking.