First of all, the piece of shit in question is still your husband's friend. And while you don't owe POS anything, you做owe some degree of trust and confidence to your partner. Assuming your husband has not given you the all-clear to go blow up his friend's marriage, you should not do it behind his back. Whatever you do here will inevitably impact your husband's friendship, so he deserves a say in how you proceed.
And that brings me to the most important point here: The ideal candidate for this job is the man who actually had the affair.他应该是告诉她的人，特别是考虑到他们现在期待一个孩子在一起。如果您想在这种情况下进行干预，我会将您的精力引向他，而不是她。
I can't promise that he will listen to you — as we've established, he sounds like a piece of shit! But either way, you'll have done your part by urging him to admit the truth. Given your position as a relative outsider to the situation, I think that is all you can reasonably be expected to do. The rest must be up to him.
You're a good person for wanting this woman to know the truth about her marriage. And for better or worse, I do think she's bound to find out soon enough. If your coworker told you about the affair, it's safe to assume she's told other people too. The word will inevitably get to the wife — I can only hope her husband gets to her first. She deserves that much, and a whole lot more.
