My approach would be the incomplete truth approach. I would tell her that I spoke to K and he told me that he and his wife are having problems, and that she is talking about getting a divorce. Then I would move into my opinion, and talk about how we should support our friends' marriages, and what a bad thing divorce is for a family, particularly that family. She might suspect that you know she is already not agreeing with you, but she won't know. Then you see what she says. Will she give you any info? Will she say that she has talked the K's W or will she feign ignorance?
This allows you to get your message across about what your role as friends should be and see if she will talk honestly to you.
