Yuk, this is a damned if you do and damned if you don't situation. Here's the way I see this one. If you are close with your friend, you sit her down and prepare her, tell her you have something to tell her that is going to upset her. Tell her the when and how's of it and how you came to know of it. Understand though that depending on the strength of your friendship, this could backfire on you.
I told a friend of mine once that his fiance was cheating, how did I know, the idiot herself told me? Like I was going to approve or something. I told him how and where. He came to me three days later and told me never to tell him anything like that again, because he was sick of people ganging up on her. The only person ganging up on her was a co worker and the mail man......lol.
Anywho, you tell her, but tell her why and how you know. Telling her that you know is simply not good enough. Then tell her why you told her, and that is because you are best buds and that you don't want to see her treated that way and that you would want her to do the same for you.