I'm just wondering what the dynamics are of the circle of friends itself? Is this your daughter's set, which incorporated her neighbor friend, or vice versa? Sometimes the undiscussed disparity of social clout/capital within a group can cause problems, esp. if they were close and "equals" before. One may be pulling away a little bit, resentful of the other 'riding her coattails' while the other is also equally resentful about the change for her own reasons.
I've worked with girls up to about this age and have noticed that by the time they are leaving the primary grades, the social strata can radically alter relationships. All sorts of things can factor into this: boys, commitments to personal interests (think athletics and dance, music), commitment to academic studies (If one is enthusiastic about learning/work, it may be off-putting to a friend who has to struggle or thinks this isn't cool) and even differing levels of maturity (One still enjoys 'kid stuff' while the other is crazy about the Twilight world). All this to day that there's so much nuance, it's not easy to figure out.
Someone recommended a title that I'm going to check out, and I'll throw out another one here: "How to talk so kids will listen...and how to listen so kids will talk" by Faber and Mazlish. This excellent book gives tools which help us start conversations with our kids which allow parents to empathize and allows kids to work out their own problems, as well as find their own solutions. This can be empowering for both you and your daughter: it acknowledges that the situation really isn't Your Problem and entrusts her with your faith that she will eventually figure out a solution that she can live with.
So sorry about all this~ girls can be so hard on each other! I hope you and the Mother of the neighboring household can keep your relationship friendly in the midst of all this. Be there for your daughter and let the girls figure it out. If one or the other has grown out of their friendship, better to find a way to be civil and move on than to force something neither may want.