I have been on both sides of something like this. I definetly understand and empathize with that physical reaction. Clamby hands, gross feeling in the pit of your stomach, heart pounding, ETC.
However, I happen to believe that if you think about it in a more calm state, you'll come to realize that someone online who lives so far away likely isn't trying to "create an in" for herself with your bf.
Of course you're gonna be upset by, and not agree with her saying your bf should leave you. Abandonment is every BPD person's worst enimy, especially by a loved one.
but did you consider that maybe she gave him this advice based on something he told her?
I'd really like to help you settle this internal conflict, feel free to PM me if you would like to talk about it.