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Hello and thank you for using JustAnswer. The best thing for you to do would be to use examples of her behavior towards others in the past and how it was problematic. Also if you have mutual friends of this former friend and you, than you should try to invite him/her with your current friends and then most likely a conversation will start about this former friend and hopefully it will turn negative. Also another tactic is to plan outings with your friends that engaging and fun, as this would help them to be closer and more attached to you.
You cannot stop them from making a choice or from listening to what others have to say about you as that is beyond your realm of control. You can only control how you behave and act with them. What they decide to do is on them, just try to continue being yourself and maybe employ some of the tactics I mentioned above.
There is no appropriate way, it is just a trial and error approach in how you confront the person. BPD individuals do not have a manual that tell others how to behave around them, they are mostly unpredictable.