I take it that it is the younger daughter having the problem?
It could be that as they approach the end of middle school, your daughter and/or her friend have developed separate interests, and simply don't have as much common ground as they did when they were younger. It's not uncommon for teens to grow apart from elementary-school friends. It's also possible that one values the friendship more than the other, and feels threatened when the other pays attention to other friends.
It sounds like you're friends with the other girl's mom, so I understand why you value your daughters' friendship. It may be, however, that the best you can hope for is that they're civil to each other when you get together with the other mom.
Instead of digging through the she-said she-said, you should talk to your daughter about the friendship she's shared with this girl, and what she's valued about it, and whether/why she wants to keep it.