Catching a cheater

If you know that a married man is cheating on his wife do you think she should know? Whether you are unfortunately the other woman or just a friend etc.....
Or do you think that is their own business and if she finds out on her own is up to her. 
Would you want to know if your SO especially a husband were cheating, having a full on affair? And to what extent would you want to know? Just that it's happened or how far it has all gone?