Got an Answer?
You're friend is a shitty person. I would find a new housemate and ditch her. She has put you in a compromising position where you know everyone involved and when the shit hits the fan (and it will) it could make things very uncomfortable at your work!
Tell HIM that you are uncomfortable keeping their secret. Let him make the choice to take their affair away from your presence, or have you tell his wife. He might decide it is not worth the risk and dump your “friend”, or it may be the catalyst to him leaving his wife. Either way, you have declared you no longer want to be in the middle and you have removed yourself from it.
To be honest I don't think losing your friends would be that big a deal. I agree with the the other posters, she is a terrible friend and has put you in an awful situation. I would tell them you are uncomfortable with the situation and you want her to move out. If you were in the wife's situation, would you want someone to tell you or find out later that everyone knew and no one bothered to say anything? Personally I think that you are more morally obligated to tell the wife.
Tell the wife, she deserves to know.
Imagine how you would feel if it was your husband and everyone knew.