Again, I have to ask: Why? To what end?
But this time, I'll answer myself: To satisfy your idea of what it is to be nice to people. You are clinging rigidly to that for some local moral crusade, despite it's efficacy being brought into question with some bloody good arguments and examples in my opinion.
Do you not think, having read some of the potential problems of this course of action (telling the guy up front, of the conversation with the BL, before she gets to speak to him), that your moral compass may be just a bit on the blink here? Can you not also see that a more neutral course of action would avoid those problems? Will you not entertain the idea that there is also a moral responsibility to the BL who of course would not expect it to go any further.
Anything anyone ever says to me is in confidence unless they tell me otherwise. I have people I trust with my life, my non-existent kids and up to half my bow-tie collection who I would not tell anothers' secret to. It's not my secret to tell. To repeat it strikes me as a touch too "inadmissable" and a whole smashing uppercut too "snitchy".
As Sir Humphrey Appleby said, "He that would keep a secret must keep it secret that he hath a secret to keep."