Every few weeks?
Could this be hormonal for the "moody" girl?
It is best to let them get on with it, especially as there is no bullying or nastiness, they will learn thier way and your DD will learn how to navigate conflict.
My year 4 DD is having a few friendship issues in a similar sense of so and so is being left out. All the girls involved are equally to blame as the other. One parent though like to blame it on the more outspoken child, which makes it trickier. It is good that you and other mums are on the same page.
I know mine is younger (and possibly ND) but we talk through role plays/scenarios and discuss how we would deal with certain situations but also she has a worry diary, which sometimes is more of a "frustration diary" where she writes out what happened and processes it in her head.
Your DD bringing it home and talking to you if probably her way of processing and getting the frustration out (a bit like us coming home and off loading after work).
It can be tedious, but I'd be happy she's still keen to tell you her worries. If you show you listen to the small stuff, she'll be more likely to come to you with the big worries.
Just ride it out. Sometimes people set a timer and you use that time to off load about your day and get it out of your system and then when the time is up, you put it aside and carry on with your evening. Is this something that work for you both? It might help stop rumination on your DD's part and make it more manageable for you?