Man this is so friggin tough. I understand people saying keep your nose in your own business, but thats whats best for you. What do you think is best for this guy?
I remember my first real girlfriend in college... she was a total asshat. But I stuck with her for two years, until one day a new friend of mine turned to me and said: "Shes a real b*tch huh? Why the hell are you with her?" And my eyes opened, I saw exactly what this chick was, when I could never really see it before (or ignored it) and I dropped her. I always wondered why nobody ever said anything to me about it before and question some of the guys at the time I thought were my best friends.
Now that chick wasn't cheating on me, and I did feel like it was a waste of two years of my life, but part of me was so godamned happy someone helped me see the truth.
I think, if you can, you oughta ask your buddy what the hell he thinks is happening when his wife is out all the friggin time. Eventually he may find out she has been cheating on him and if he comes to you then and figures out you knew and didn't tell him, he'll have a harder time forgiving you than if you at least try and point him in the right direction now.
I believe its important to help him open his eyes now so he doesn't quit wasting his time, money or life with a two timing tramp. Maybe they'll patch it up, but regardless, I think he has the right to know that hes not the only dick in that hole at night.