The Queen revealed that the King is "getting better" before adding: "Well, he would be if he behaved himself."
Camilla's comments came as she chatted with guests at a garden party at Lamb House in Rye, where she was treated to a performance by actors Timothy West, 89, and Hayley Mills, 78, while sat alongside Prunella Scales, 91.
The Queen, 76, said that her husband, 75, was "quite cross" that he couldn't join her at her engagements in East Sussex on Thursday, as he was a big fan of author EF Benson, who used to live at the property. The King was otherwise engaged with audiences at Buckingham Palace.
Charles resumed his public-facing duties earlier this month after making encouraging progress in his cancer diagnosis, although sources have previously stressed that despite the welcome news the King still has cancer and will continue to be treated for the undisclosed form of the disease.
Camilla visited the county in South East England to celebrate literature, earlier revealing at the Charleston Festival that the one fictional place she'd love to visit from a book would be Hogwarts.
Watch the moment in the clip below…
The Queen's outing came just hours before the King surprised the audience at the Royal Opera House as he appeared on stage hand in hand with performers at a gala celebrating the conductor at his coronation on Thursday evening.
The event paid tribute to Sir Antonio Pappano at the end of his 22-year tenure as music director of The Royal Opera.
Performers were told Charles would be attending 20 minutes before they went on stage, and their show closed to a standing ovation. After the curtain fell Charles and Sir Antonio spoke about the time that has passed since the coronation last May.
Sir Antonio said: "He was just so generous in his praise, and genuinely so, and that means everything, he loves music. He knighted me, and I've done several events at Buckingham Palace over the years where he’s been the engine for those things, and of course I conducted at the coronation, so we’ve done quite a few events."
The King's jam-packed diary
It has been quite the week for the monarch, which began with him handing over a military role he had retained for 32 years to his son, the Prince of Wales, at the Army Aviation Centre in Middle Wallop on Monday.
Prince William has become the new Colonel-in-Chief of the Army Air Corps, carrying out his engagement with the regiment after the official handover ceremony.
On Tuesday, Charles unveiled the first official portrait since his coronation, with the striking red painting by Jonathan Yeo met with a mixed reaction. The piece showed the King wearing his Welsh Guards uniform, with a butterfly hovering by his shoulder.
The British artist explained: "The red was inspired by the Welsh Guards, but I wanted the painting to be a little more contemporary and not get in the way of seeing the face and the personality.
"The colour was an early experiment and then I sketched it out and worked on the face, and the face and background worked so well. I just then worked on making sure nothing else interfered with the balance. It was a nice mix of the traditional and the contemporary."
On Wednesday, the King and Queen attended a Service of Dedication for the Order of the British Empire at St Paul's Cathedral, before mingling with guests at a Buckingham Palace garden party where Maya Jama asked Charles if he watched Love Island.
Chelsea Flower Show
Charles will kick off next week with a visit to the Chelsea Flower Show on Monday, Buckingham Palace has announced.
He will tour the gardens and view the displays in London with Camilla, accompanied by the Duke and Duchess of Gloucester.
The royal visit to the RHS Chelsea Flower Show the day before it opens to the public has been a regular fixture in the royal calendar for many years.
Charles is the new patron of The Royal Horticultural Society, following a review of royal patronages after the death of Queen Elizabeth II.