Tell your wife that she should write that letter.  It will be good therapy to help her get her feelings out on paper, and not keep her feelings bottled up in herself.   Write that letter......  and then BURN IT.  Under NO circumstances do you send it.  Write the letter and burn it - it will be good therapy.

In the past I have written letters where I needed to felt my feelings.  I have laid that letter aside; a few days later, I reread what I wrote, and then toss it.  It was a valuable process.