I would venture a bet that you have a lot of friends who've cheated who never told you, or anyone else close to them for that matter. In fact, if 4 of your other friends have crossed the line at some point in their lives and just not disclosed it you'd be at 30%.
I think this study seems very possible. Heck, the divorce rate in this country is 55%. I see a few divorce cases every year, and I'd say that in 95% of them at least one of the spouses has cheated, or been accused of cheating at some point in the marriage. So to conclude that one in three women with children cheat on their husbands at some point in their lives is probably not too far off track.