B.C.
It's true that kids self regulate their own friendships as they get older.
That being said - not every kid takes well to social media/cell phone usage - some just can't handle it till they are older./more mature (although unfortunately some never mature no matter how old they get).
I think you need some quality one on one time with your daughter where you both have a weekend with no phones (go camping, take a weekend trip, etc), and do it periodically.
You and your daughter need to TALK - about how she feels about her friends (all of them), what makes her happy/angry/sad/mad, how it's good to have a wide circle of friends - because they all come and go over the years and through a lifetime.
Talk about her goals in life (it's not too soon) - about what her favorite subjects in school are - you are her mom but you are her first guidance counselor.
She needs to be involved in some after school activities where she can meet new friends outside of school.
Middle school can be tough - it's a big transition - and friends can be hard to navigate.
While I wouldn't forbid her best friend - I would tell her that you DO monitor her computer/cell phone usage/content - and actually do the monitoring - don't just say you will.
Texting at all hours of the day and night is not acceptable.
Unlimited access is never a good idea - have your daughter earn her perks with good behavior (chores, homework, etc) - and if she doesn't earn any then she gets none.
You can't control what the other girl does but you can control your daughters access to devices.
You take the phone/computer away (and lock them up) during the hours it's not acceptable - like over night, meal times, family time, when she's driving (it's only a few years away - get in some good habits now well before she gets a learners permit),etc.
Your daughter needs to tell her friend that she is not available 24/7
It could very well be the other friends do see something undesirable in this best friend that your daughter is blind to.
Maybe they are being mean but it's not necessarily so.
It's difficult being around a major drama queen - the only thing for it is to walk away and the other friends are managing as best they can.
It's not up to your daughter to fix her friend - some people try to.