Yes they would need to know that their involved with a liar. And their partner doesn't deserve the friend anyways! :) but it depends how you tell them.
I learned to do it in a certain way that works for me. I would want to know and have someone tell me in even an awkward way. I think cheaters belong in the same class as murderers; they are both killing something very precious for no reason. I will ask my friend if that was their husband/wife I saw at the movies talking/with...? Then that is it. I've done my job and it is up to my friend to check the story and make any further decisions.
Yes, it would be hard but i would tell my friend, just as i would want my friend to tell me if the situation were reversed
Yes, I would. If it is my friend that is getting cheated on. If I don't tell her/him then I am not being a true friend. What really sucks is if you are friends with the both of 'em and one is cheating. Who do you be loyal to in that situation? Tough one!!!
Yes. I believe they have a right to know. And if the partner won't be honest, I will.
Yes, I will.
Probably yes and I would ready myself to lose both of them as friends in the end (that is usually what ends up happening when the couple makes up one way or the other).