As a true friend, it'd be best that you did. If you know for fact he is, and can back it up with some sort of proof then keeping it from her will only hurt her more in the end. She may feel betrayed...
If she is a true friend, you should not tell her husband that she is cheating. You should do everything you can to help her keep it a secret from her husband, including lying for her. Real friends...
YES!!!! ANSWER: If you think this will stop what they were doing, do so, but make sure that you have some good evidence that will back up your story. If not things might go bad for you. Good luck!!