Um, I think it's best not to say anything, like Jo said, if you and your Dad know then the chances are others do too.  She may already be suspicious or heard something herself.

 

I'm glad you didn't say anything to the wife.  She probably knows full well what a louse she is wed to.  Telling her forces her to take a stand which she is not likely ready or willing to do - perhaps now, perhaps never.  Some people just look the other way because there is so much at stake financially and for any children involved. Who knows? Maybe she appreciates that she doesn't need to snog the prick herself!!