It's a tough situation. No matter what you do... or don't do... you run the risk of one or both of them getting angry with you and/or your wife. If you tell the husband, it's a certainty that his wife will blame yours. And your friend may not believe you either... thus be angry that you'd even suggest that his wife was capable of cheating. And if you don't tell him, then later he may blame you because you didn't.
Kinda think in your place I'd do what Elsid suggests...keep my nose in my own business and hang out with them a little less.