Eve, find yourself a counselor. Open up the yellow pages, flip to "counselors", call one (preferably a woman) and make an appointment. Make it a long one. Spill your guts to her and then go from there. Yes, it will cost money. If you have insurance through your employer, that should cover...
Well, Eve, I can't make the decision for you. But I can guide you to one. How is your relationship with this couple? Are you very close friends, or is it a surface relationship where you enjoy each other's company and nothing more? Or maybe somewhere in between? How about respect? Do you...
You didn't answer the most important question: Do they have children? If so, get the hell out. You don't need to be a player in wrecking a kid's life. If not, then you have options. I recommend ensuring his wife is OK with it. Not, "Oh, she sleeps around too, so who cares?" If she knows you...