Yipes, that sounds like a difficult situation. You are basically at their mercy. Unfortunately I don't think there is a good way to get rid of him I think you are stuck with them both really unless you want to make a scene, which I believe will end badly due to the possible outcomes.

I have these rules I stick to - when it comes to confronting people about their behavior in a professional setting (same stuff happens with co-workers for example). No good comes of talking about it. Just don't. This is my main rule, which isn't always practical, obviously, and so there are some other rules after... er. I am not saying it should be your rule... but it is my recommendation for you. Once you confront them you let them know that they have a way to get at you, and really you can't expect they will leave. Further, because of BPD problems with how we view and relate to others they somehow may not be aware of their behavior and may even have a completely different feeling than you - anything you do will come out of left field for them, and if it doesn't they can pretend it does - either way it is win-win for them.

I would revert to trying my hardest to roleplay the naive and open communicator. Kill them with kindness - beyond that of customer service. Don't even try to 'play back' with either. You could try scaring them with brownies or something.... ok that might be a bit much (was really joking).

I wish I had some help for getting rid of them. Perhaps try talking of things other than your field. Doing a DBT worksheet with possible outcomes and possible alternate methods of coping might be helpful. Do you have a DBT self-help book or are you familiar with any of the DBT skills?

How do you eat your lunch, btw?!