Chrissy Hart, whose daughter passed away this week, said Rowling's novels "built Hogwarts as a home for her to return to" in the letter, reported BBC.
"I write to you as a mother, a Mummy, of a beautiful little girl. I'm sure you hear people tell you all the time that your words, your imagination helped them, or their children, in some way.
"I want to share with you what you taught my daughter, who was recently diagnosed with cancer...You taught her that for every chemo she has to go through, there will be the opportunity to fly across a lake on a hippogriff. For every time her heart seizes with fear of pain, there will be me chasing it away with a patronus charm.
Hart's letter was read by actress Ellie Bamber, as part of the latest performance of Letters Live, a series of performances in which remarkable letters are read aloud.
"But the most important lesson you have taught us both: cancer may take her away from me in just a few short weeks. I might not be able to spare her the pain. But for every child that is taken too early, people will come together, huddling close in a big group, wands held high to light the dark sky with the love they felt.
The letter was written last month.