Being a good friend means telling them when they are doing something wrong even when they don't want to hear that. As a friend, I would tell her, you will not lie for her to anyone. And not to put you in the middle of something. Tell her, she's your friend, very good friend, however, what she's doing to her husband is wrong. No matter what he did to her, she made a vow to him, and most likely "for better or worse". Even though he cheated does not giver her the right to cheat. I would also just tell her that you don't want to know anything more. Even if she found another guy, you think what she is doing is morally wrong and you don't support it. It may hurt her, but it will let her know where you stand. She won't be telling you things.
I don't feel it's your place to tell anyone. I feel lying is wrong, plain and simple. If your friend knows you aren't going to lie, she shouldn't put you in that position. If her husband asked you anything, I would tell him to go talk to his wife. Everyone is grown, you handle your own business. If your boyfriend ask you, I'd say that's their business and you are not part of it. Don't let your friend guilty you, she's an adult, and responsible for her actions. An adult doesn't make you right for doing what you want to do, but it should make you accountable for your actions. She knows the difference between right and wrong and is doing wrong for the short term happiness and long term regret. I would tell her that too. Let her know, her actions today not only affects her, but her husband, daughter, etc.
You are a good friend, don't let her guilt you!