This poignant, truth-filled letter is an example of exactly why I keep coming back to this board for strength and encouragement. It's "food at the proper time" for those of us who think and feel.
The parents' reaction will depend on how much thinking and feeling they allow themselves to do.
I got a similar letter from my own daughter many years ago. I was furious, of course, and really hurt by it; but I didn't throw it away. I created a manila file with her name on it and tucked it away in a metal cabinet drawer. Over the years, I came to realize she was right. It was just one of many, many building blocks that have combined to free me from what I now realize was mental and emotional enslavement. Today, we have a warm and mature relationship based on unconditional love.
Your friend has a lot of guts, has taken his/her life back, and is an inspiration to us all.