Difficulty: Moderately Challenging
Step1
Aknowledge that her husband is in fact cheating.
Step2
Have her read the article "How to face a cheating husband and the other woman without losing him"
//www.ehow.com/how_2255432_husband-other-woman-losing-him.html
Step3
If she has done these 2 things, and still wants to stay with him, then you will need to mind your own business and be supportive to your friend. It's her decision.