Sarah Ferguson urged the King to "keep fighting" during a moving speech, as Prince Harry shunned his father in Las Vegas.
The Duchess of York took to the stage in Palm Beach, Florida on Wednesday to give a speech at The Breakers Hotel.
Fergie, as she is affectionately known, was attending the Cancer Alliance of Help and Hope's Palm Beach charity event.
During her impassioned speech, Sarah shared a touching tribute to King Charles as she thanked the charity for supporting her family.
Sarah Ferguson wished King Charles well during a speech
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Both members of the Royal Family were recently diagnosed with cancer, with the duchess's being skin cancer.
Sarah said: "I want to thank you all for your kindness in supporting my family members who are going through a hard time, and we wish everyone well, every family in the world, that has been diagnosed or who is going through a difficult time we wish them well.
"And of course, my brother-in-law, we wish you well, keep fighting on."
The words were recorded by an audience member and shared on social media.
Prince Harry photographed at the NFL Awards
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The duchess added: "The cancer alliance is about hope, and there isn't enough hope in the world.
"For anyone who's suffering from low hope, for anyone who believes they can't do this, they can, it's absolute nonsense."
It comes as Prince Harry, 39, took to the stage in Las Vegas on Thursday evening where he did not publicly send good wishes to his father.
The Duke of Sussex turned up to present the NFL Walter Payton Man of the Year award.
Sarah Ferguson photographed in Miami earlier this weekGettyLATEST ROYAL NEWS:
King Charles has been diagnosed with a form of cancer
PAHarry is understood to have flown to Vegas 24 hours after landing at LAX Airport.
This followed an 11,000-mile round trip to London to see his father, King Charles, for 45 minutes at Clarence House.
Earlier this week, Buckingham Palace confirmed the King had been diagnosed with a form of cancer, which was discovered during surgery for an enlarged prostate.
The same day, the Duke of Sussex was confirmed to be travelling to the UK to see his cancer-stricken father.