ok been in that position and honestly the best thing to do is say nothing at all or you put yourself in a horrible position

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I love the night
I love the element of danger and the ecstasy of flight,
I love the night
I love to dance with a stranger and to feel his delight,
And when the dancing is through, I kick off my shoes,
And I listen to the beating of his heart

Are they married? Yes about two years

Did you know the cheater or the cheated first? I dated the non cheater several years ago. We have remained friends (attended their wedding etc).

Does the cheater know you know? Yes he knows. He asked me to cover for him so he could meet is new 'love interest'....yes i declined to participate in the ruse

Do you all hang out regularly? the cheater is a guy who i go mountain biking with sometimes

How long have they been in the relationship (if they aren't married)? Married about two years. Dated about 2 years before that...

Do they live together? yes

Are you close friends with both...one more than the other...and for how long? I am not close but know them well.

thanks all for the advice

Stay out of the situation! Relationships are complicated things, and cannot be judged from the outside.

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Life is too short not to love and be loved....preferably multiple times in one night.

I think men talk to women so they can sleep with them and women sleep with men so they can talk to them. ~ Jay McInerney

Are there kids involved should be the only thoughts you keep to yourself.

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I hate your situation.

But, the only way I would say something is if the person being cheated on was a really close and dear friend to me. Bestest friends don't keep secrets of this magnitude from each other.

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Minds are like parachutes. They only work when they are open.

~Thomas Dewar~

I tend to agree with the others - it's a really difficult situation to be in. I've been cheated on and I suspected (and was right to) that some of my friends knew about it at the time. I wished that one of them had been honest with me, as I felt like a complete fool when I found out.

It really isall dependent on how well you know both parties, and how you feel morally about it. I feel really sorry for the person being cheated on more than anything - they obviously have a right to know what's going on, but it's how comfy you feel about being piggy in the middle.

I think a quiet word in the shell-like of your cheating friend might be called for, even if just to let them know how uncomfortable you are with the situation and how you do not want to be party to it. If he/she really is your friend then they will respect that.