It is very difficult, for each circumstance is diffferent, and each friendship we may have had as witnesses are different as well.
If your friend is sincere, then perhaps she is doing what she feels is right.
If you are going to just fade, then I would be very vague in anything I would say. If you are going to DA yourself, then absolutely a letter stating exactly why would be the right thing to do. Who knows, perhaps you will reach her heart, if not now, later.