I would tell her anonymously (with proof, if possible) but not face to face. I know from a prevous bad experience that you can never tell who would want to know if their significant other was cheating and who would resent you for telling them the truth. I used to say that anyone who would 'shoot the messenger', so to speak, was not worth having as a friend but I have since learned that there are A LOT of people that don't really want to know the truth. (I told a friend that his girlfriend, my co- worker, had been bragging about sleeping with other men and it blew up in my face.)
I think it makes things more difficult for the injured party when other people know their sig. other has cheated. When everyone knows, it is expected that the person will then do something about it- kick the person out of the house, ask for a divorce, go to marital counseling... It's almost as if they feel pressured to respond in a certain way or risk looking like a fool. If the person was told anonymously, or if they found out on their own, they may choose to forgive and forget or ignore the infifelity. But when confronted with that information by a friend, people often begin to resent the people that tell them instead of taking it out on the cheating partner. Sad, but true (at least, in my experience.)
