How do you treat a friend, who been lying about cheating on their spouse?

It depends on a great deal, how do you feel about the whole situation, are there extenuating circumstances that would cause you to feel your friend needs your support? Do you feel you can't trust this friend any longer because you see how well she is at telling lies? Do you wonder at what things she may be lying to you about? Is she a real friend, one who has stood by you through the hard times in your life, one who has given you the support you needed when things were rough on you? Or is she more of a casual friend, but not someone you would call on in a crisis? All of this can determine what you should do, or how you should treat her. If you don't feel that there is trust between you then it is time to walk away, If on the other hand there are extenuating circumstances and she has been there for you and supported you when things were hard for you, and you feel you can count on her to still be there for you, then let her know if it bothers, and where you stand, be honest with her but be there for her.

tough call i can say just let her keep lying she'll get cought soon enough

I would talk with your friend and tell her that you don't want to be in the middle anymore. I would be up front with her that if she continues to lie to her husband, you feel that you cannot continue in the friendship. Make it perfectly clear to her how her lying and covering up makes you feel. I wouldn't go to the husband. It is better to stay out of matters between husband and wife, but I would discontinue the friendship, if she isn't honest.