I'm not sure you need to make any kind of announcement or explanation to the other parents. If the girls are uncomfortable being together, that's all you'd need to say if the other parents should ask why their daughter isn't spending as much time with you - "The girls aren't comfortable together any more. I think they'll both be making new friends." No accusations or ultimatums or explanation, just simple statements.
To your daughter, I'm sure you've already given her the "growing up is hard, and some people go through difficult phases" talk, so she knows that she did nothing wrong, but that it also isn't in her power to fix it. It's hard when a friend decides they no longer value the relationship the way you do, but all of us go through that, and learn valuable lessons from it.
Just as an aside, don't become so hostile that you write off the girl and her family entirely. My sister and I were best, best, best friends, from the time I was 3, with the pair of sisters who lived across the street from us. We were together every day, our parents socialized together, we did things with both families together, my older sister babysat and housesat for them. About the time we hit junior high (6th to 8th grade, for us) we were no longer as close. There were no arguments or obvious slights, each of the four of us simply made new "best" friends, so the other friends became secondary. We still liked each other, we just spent less time with each other and more time with other kids. A couple of years later, the family moved across town, and we saw them rarely. Now that we're all adults (and living in different places, in some cases in different countries) we keep in touch mostly by letters, cards and e-mail, but we still treasure those friendships. When my daughter got married last spring, those friends were there to celebrate with us, even though we hadn't seen each other for years and it meant some significant travel for them, and we were very touched. There might be another chapter down the road.
If not, that's OK too. Many friendships don't last into the teen or adult years, and that's OK too. Sometimes working it out means ending the relationship before severe bad feelings or harm takes place. Think about kids from your childhood, and how many you're still close to.