it's my first post on this thread... but I think the right thing (for your conscious) to do here is to tell her.
That would make you feel easier. But most likely it will ruin your friend's relationship or at least put it through a serious test and likely create tension among you three.
I'll tell you about my previous situation, it differs a bit though.
I'm a guy and when I was in a similar situation (I didn't turn her down b/c she was my dream girl), I thought I owed it to my friend (who has been infatuated with the thoughts of her) and I told him what had happened. And well that girl became very angry with me for "hurting him" and well my friend is no longer my friend after he went through about 3 months of depression.
but it really depends on the personality of those involved in this issue... if it matters, my vote is to not to tell her, for the sake of those two and rather talk to the guy about this and see how much he cares for her... if you want to protect your friend.