You are the ethicist!

How would you answer this?

Five years ago, my dear friend A. was caught by surprise when her husband asked for a divorce. She was shaken, crying and in disbelief for months. Now, just a few months after her divorce was finalized, she has met an old boyfriend who is married to another woman, B. The women know each other, and A. has now told me that she will be married to this old boyfriend within the year. She told me that this man has made plans to announce to his wife that he wants a divorce. B., who is an acquaintance of mine, is completely unaware of the impending disaster; she thinks her marriage is fine.

I can think of several roads to travel here: One is to confront A. and remind her of her pain and incomprehension. Another option is warning B. about the brewing storm (and I imagine she will take actions to save her marriage). A third is to keep my mouth shut and record the disaster as a morality play. Am I ethically obligated to admonish A. or warn B.?

You can read the Ethicist's response here.