In my opinion if you are sure about what you saw then you should tell your friend... If you think you shouldn't say anything because it might ruin her marriage then think about this if your friend suffers in any way and becomes un happy with her marriage you would know about it any way.. and then you will tell youself that you could have done something... from experience I am saying that marriages are very hard to recover from if a couple gets seperated..  So ask you conscious and think what would be best for your friend aand then act accordingly..