My best friend's other best friend is so clingy to her.?

I just moved to this new town and I have made a best friend. Her name's Brianna. I have met her family and they all love me 2 death. But anyways we are super close and she has her other friend she has known scence, idk 4th grade I think bri said but anyways and scence I came and became closer to Bri then she will ever be, she dicides to start clinging to Bri and everytime I try to make plans with her Dianna is right there listing. When Dianna isnt at school, Bri complanes and complanes yet she wont tell Dianna that she doesnt want to be her friend, she even told me that she didnt want to so I dont understand why Bri dosnt just tell her. What should I do to help Bri make the right choice? All i want is her to be happy with her choice and I know if she decides to stay friends with dianna that she will regret it.