To tell or not to tell

You've done your part telling your friend of the situation of the guy. For me you are not in the position to tell her family just don't feel so guilty about it.

I am brand new to this site, and something led me to your posting here. I can tell in the way you have written this that you are very concerned and sad for your friend. What is happening is unfair, not only to the wife of the teacher, but also to your friend. Most of these situations never turn out well, and she will end up feeling used and betrayed in the end. I had a friend in a similar situation once and unfortunately, the married man never left his wife or family for her and she ended up wasting a lot of time on him, just waiting. While she was waiting for him, she missed opporutnities with other people who could have turned out to be the right one. She learned the hard way, and it is so sad that this happens to so many people out there. The guilt that this causes will start to really harm her, not only emotionally, but can start to affect her physically. And you are experiencing guilt too for knowing about the relationship and not feeling like you can tell anyone. It sounds like you want to do what is best for your friend, and I would definitely pray about it and ask God for the right words to say to both your friend and her family. Adultery not only harms the man and woman involved, but also the rest of the family, the children, and the friends.