Dianne.....Everyone has her own opinion on this awful situation so heres another one to chew on.
Personally I would never be the bearer of this kind of news, the wife will though possibly grateful to know will never forget you told her,and neither will the cheaters. The best solution if you are unable to know this secret and keep silent is to send the wife an anonymous note. I wouldn't mention any names just that if shes smart she will either hire a detective for proof or follow the asshole herself. Then you can have a clear conscience. You gave her the option to pursue this if she chooses to or not! Killing 3 birds with 1 stone so to speak. Mybe one day I'll tell you how I saved my cousin and best friend Arlene's marriage many years ago.