Re: moving on
In my opinion, give your friend the letters. Moving on is a process and your feelings can't be turned off like a light switch regardless of whether you remarry or not. Your friend is an adult, and she's aware of the fact that she has remarried and about the demands and expectations of a marriage. Regardless of whether you remarry or not......you can move on....BUT moving on DOES NOT mean that you will FORGET your previous relationships. You can move on and with time the pain will lessen and you will heal....but you will NEVER forget someone you loved and shared a life with.
Those letters might even help bring your friend CLOSURE. After the death of a loved one, sometimes people are still in denial that the person is gone. And having those letters can, the letters could perhaps change denial into realization and bring complete closure.
Also, those letters are her possession/amanat and if she wants them, they should be returned to her. I understand you care about the well-being of your friend and you can express your concerns to her.....but in the end it's her decision to do what what she feels with the letters that belong to her.
And also, her previous husband, in the pain/heartache/agony of cancer, nevertheless took the time out to painstakingly write those letters to her. And after all the energy that he put into writing those50-50 letters.....they should go to the rightful owner....especially since she has expressed the desire to have them.