cowards behind email

Today I learned someone had been posting malicious things about a friend of mine, even going so far as to set up a gmail account in his name. After I read through some of the posts, I realized I knew who this was. Now the dilemma is --what do I do about it? Do I let her know that I know? Do I expose her to our friends? Do I tell him? Now I love the internet but suddenly, coldly, I realize how dangerous it can be. I'm a big girl and can take what comes --face to face, but how do fight a bundle of anonymous lies like this bitter old two faced woman is posting? What if she turns her vitroil towards me, hiding behind the blinking cursor like the coward she is. Anybody got an ideas on this one?