Hi Kaylord,

As you are probably aware, the first step to any problem is recognizing there is one. This is kind of a delicate situation, because people with BPD, we can never really know what will or will not cause them react. Thus, the feeling of walking on eggshells.
The reason of her jealousy is probably not about possession but out of fear, an intense fear of abandonment as it seems. Just so you understand, people with BPD don't actually need any proof of possible abandonment, they most likely will go to extreme measures to avoid it if they suspect a possibility of being abandoned or replaced. Whether that possibility exists or not is almost irrelevant.
I think its a good thing that you want to intervene, but you have to do it with care and there's a certain limit of what you can do. I say that because she doesn't seem to be aware of her "self sabotage" behavior. And if it goes on, it is possible she goes to other measures to make sure she feels understood. After all, the reason she feels the need to do this is because she doesn't feel good about herself and probably has some doubts about herself deserving to be loved.

I would suggest to start slow, you know, talking about how she feels and why she does the things she does and perhaps leading her to question her own behavior to eventually make her realize what she is doing. It has to come from her, all you can do is lead the way.
Now I know it is different from person to person but I believe childhood plays a major role in BPD so if Michelle wants to help, I think she could take some time to talk about it with Diane, you know, anything to help her come to realize.
Anyway, I really hope this helps! Its very nice of you to want to help your friend, if you have other questions, feel free to ask !