Hi Uncle Rico,
Welcome to the board ,
I'm sorry to hear about what has been going on in your relationship. It's scary to think about how your pwBPD may respond to you setting boundaries with her. For me I've found that if I am open, authentic, and honest with myself before approaching my H with BPD, then I can easily, directly, and firmly say hard things. He may get mad but at least I know that I have shared with him what I need from him.
For instance in this scenario, you might try sharing with her that you are hurt from the affair and that you have concerns about her starting (or remaining) in an affair on facebook. Maybe you could come up with some ways of accountability, such as allowing you access to her facebook account. If needed, perhaps a mediator or counselor could come in to help with the conversation.
Here is a link to one of our workshops on setting boundaries. I've learned that setting up the boundary sounds hard but often my pwBPD responds extremely well to them.
https://bpdfamily.com/message_board/index.php?topic=61684.0