This happened to my ds2 when he and his best friend went to secondary school. The boys had known eachother, and I had been friends with his mum, since they were 18 months old, and had been through playgroup and primary school together.
Sadly, not only did they drift apart, but in Year 8, the other boy started bullying my son, which was incredibly hurtful for ds2. My friend also did not believe that her pfb would do such a thing, so it did cause some stress between us, but we decided not to let it affect the friendship.
I contacted the school, to get them to tackle the bullying issue, and I encouraged ds2 to make other friends, and take up activities that the other boy wasn't interested in; and the other boy totally stopped coming round to our house. It did help that, at around that time, we found out we were moving away from there - a long way away (from the southeast to scotland) so I knew that ds2 was going to be taken right away from the other boy.
Happily ds2 now has a big circle of friends at his new school, enjoys his life and has a far happier school life than before. He and the other boy have also made up - I gather that the other boy has apologised to ds2 - and he and his mum have been to stay with us a few times, and these have been happy experiences.
To the OP I would say - encourage your dd to have other friendships, and to get involved in things at school, like the musical/play (if your school does one) or sports clubs or other clubs - she will meet lots of people and will make her own circle of friends. As far as you and your friend are concerned, I think you have to accept that your friend can't make her daughter be your dd's friend (though she should speak to her about not being nasty to your dd by ignoring her on the way to/from school etc), and you have to decide whether you want to maintain the friendship with the other mum, and whether you can put aside the differences between your dds.