As a nurse, it is not my job to interject myself into people's family dynamics. It is my job to care for the patients.
That being said, the man is a scum, and if anyone in his or her family knows about this, they should be the ones telling her.
Make no mistake about it. His wife will find out, if she doesn't already have an inkling. People have different reasons for staying with someone, and until we are living in their life, we have no idea why they do. I had a colleague many years back who had a special needs child and a cheating husband with great healthcare coverage and a good income. He was cheating, she knew it. She made the conscious decision to stay with him because she knew that she couldn't handle the child on her own, physically or financially, he made a good lving and had the good healthcare coverage. She also saved for her eventual departure from the marriage. But, she always used to tell me about all the well-meaning people who would come up and tell her that her husband was having affairs. She would say "I know" and then get the pitying look. She did not need pity. She had made a conscious decision for the well-being of her child to stay in the marriage until such time that the child was settled (which took about 7 or 8 years) and during that time, she did a lot of creative saving to prepare herself for her single life. Her husband treated her nicely in that he never yelled, wasn't abusive, didn't deny her or the children anything they needed. She actually said they got along great as roommates. She made it clear to him that they were married in name only, which was fine with him because he had the type of job where having a wife was...desirable for the "image".
Could I do the same thing?? I don't know. Do I admire her for sacrificing her personal happines, for a period of time, for the benefit of her children...especially the special needs child...yes I do.