Addressing controlling/toxic friend’s friend?

We’ll call them Friend A (my friend) and Friend B (Friend A’s best friend)

Now, A and B have known each other for much longer than I have and have long been established as best friends. I met them both around the same time, and over time found myself connecting with Friend A given we’ve had more opportunities to hang out and get to know each other over a few years.

I’ve tried being friendly with Friend B in the past also but she hasn’t been very receptive or interested in persuing a friendship so I’m fine with just being acquainted on the basis of having mutual friends such as Friend A.

The thing that’s been bothering me is that more and more often lately I’ve noticed Friend B seeming to both passively and actively trying to get in the way of mine and Friend A’s relationship. At first I brushed it off as being a “well best friends take priority and it’s nothing personal towards me”, but more often things gradually became less excusable.

* Friend A invited me as a +1 to an invite only movie screening – Friend B (who had said before has no interest in the movie, compared to me who has been hyped hence the invite) finds out less than a day before and demands to be the +1 instead.
* Friend A and B are working on a project and looking for others who may be interested in joining – A is welcoming when I express interest, B significantly less so, and casually mentions “if I see you in our group chat I will personally kick you out immediately”
* I make dinner plans with Friend A and a few others for after a show we’re attending together – friend B also turns up. After the show B makes up a new dinner plan for everyone, but only waited until most of the group had headed to the restaurant to tell me that “she wants it to just be her close friends” and asks me to leave.
* On several occasions I’m in a discord VC including Friend A, then B hops in, talks for a few minutes then asks the “people who can” to join her in another room I don’t have access to talk about “other stuff”, abruptly ending my time hanging out.

I’m certainly hurt and taken aback when this happens, but I’m also genuinely concerned for both of their wellbeing. And that’s not just based on an assumption;, there’s been a few times when Friend A has expressed frustration with Friend B’s controlling behaviour as of late in said voice calls, and several others have straight up cut off ties with Friend B entirely. Meanwhile, I know Friend B has been having a rough time of her own, having had a breakup earlier this year on top of the bridges she’s burnt, and now moving interstate pretty soon.

Friend A is a very important friend to me and we do have plans together still in the coming year I’m looking forward to, at the same time B’s behaviour is becoming less bearable and justifiable. I’m certainly not the kind of person to rock the boat and try to get in the way of other’s friendships, but I don’t want to allow this to continue the way it is, and at the moment any ideas I have to address it carries the risk of losing A and other mutual friends if I handle it poorly – I’m conscious of A’s social anxiety as well so I’m considerate of trying to bring it up in a way that won’t make her uncomfortable. This is on top of not wanting to be seen as starting “drama” or “gossip” so I gotta be careful with my words.

One suggestion I’ve received is to wait until B moves out before doing anything direct, although I feel like that may only leave things unresolved.