I could have written this a million times about every girl-relationship dynamic I've ever seen. And I studied this (and bullying, by extension) in college for my psychology degree. There is ONE solution here.
Start gently guiding your DD toward different friends. They are 7-ish right now, it's only going to get worse, especially if Friend A's mom is more concerned about her DD's popularity than your DD's feelings. She will never be on board and you will have even more problems with her as the girls get older.
I have an 11 year old DD, 6th grade, and are entering the throes of the Mean Girl stage. My DD has been friends with/in class with the same kids since 1st grade, I've been the coach of several of them, and a Girl Scout leader for part of the time as well. You can tell pretty early on who is going to end up being the mean girls, the sporty girls, the smart girls, the nice girls, etc, believe it or not. There are always the floaters who pretty much fit in anywhere, but by middle school they usually end up finding a group.
Anyway, I always thought that the "mean girls" in DD's grade were relatively nice for mean girls. The Queen Bee is smart, funny, nice, and wins pretty much everything she tries for, whether she is good at it or not. She has always been nice to my DD, and they were friends when they were little, but I started to see who she was hanging around with and how their mom's were (her mom included), and I realized that once these girls got older, I could foresee a lot of drama, bad decisions, and no support from the moms who already thought their DD's were perfect.
So, being a coach and GS Leader and knowing so many girls/moms personally, I started to (covertly) guide DD toward a different group of girls - these are the girls who are well-behaved, smart, sweet, and respect each other's opinions and ideas, but are also fun to be around. I would encourage her to invite these girls over to play, or invite them to go to the park with us during DS's baseball games, or to the mall, etc. She was still friends with the mean girls, but stopped doing so much with them after school. Pretty soon, she made the choice herself that her new friends were MUCH nicer to her, and she wasn't so stressed out all the time about being around them. She could just be herself. She is still friendly toward all of the mean girls, but she is never involved in their drama and won't get in trouble with them as they all get older.
And I am soooo glad! I am friends with a lot of the mean girls' moms (one in particular, we work closely as directors/coaches for a local team) and apparently, there was a incident a few weeks ago at a sleepover that involved a "top 10 Reasons Why We All Hate You" text that went out to one of the girls who was in their group, but someone decided they didn't like her anymore. It all blew up and all of the girls ended up in the principals office and their moms were all brought in. The Queen Bee that I mentioned before was at the sleepover and involved, but denied it and her mother actually told my friend that she "mom-failed" by even asking her DD about it because her DD started to cry and said she cannot believe her mom would think she would do something like that. Her mother ended up apologizing to HER! But all of the girls there said that Ms. Queen Bee was the one who suggested the list in the first place.
Ugg....so glad I foresaw this when DD was in 2nd/3rd grade and started to encourage her to make new friendships, or she likely would have been involved in all of that mess, too.
And, FWIW, DD invited two of her friends to a play last night that another friend of theirs was starring in, then the girls came back to our house for a sleepover. 3 girls + NO problems at all. So it can happen with the right combinations.
"Queen Bees and Wannabes" by Rosalin Wiseman, should be a must read for any mom of a girl, in my opinion. And "Queen Bee Moms and Kingpin Dads" by the same author is equally helpful. I highly suggest getting your hands on a copy of both books.
Good luck to us all. Haha