She said in the letter: "Daddy, it is with a heavy heart that I write this, not understanding why you have chosen to take this path, turning a blind eye to the pain you are causing".
Eagled-eyed royal enthusiasts spotted Meghan, 37, wearing a huge jewel on her ring finger with many speculating the piece is a second, more personal engagement token from the Duke of Sussex.
The tone of the coverage on Meghan has shifted in recent months, amid reports that the duchess has been upsetting the royal staff and whether she has fallen out with Kate.
He added ominously, "We've seen how that ends".
The Hollywood star is a friend of Harry and Meghan and he and his human rights lawyer wife Amal attended the royal wedding in May a year ago. But she's eight months pregnant, and vilified and persecuted her as Diana.
"Just seeing them broadcast a letter from a daughter to a father, she's getting a raw deal there and I think it's irresponsible and I'm surprised by that".
"You're taking a letter from a daughter to a father and broadcasting it everywhere", he continued, referencing the letter Meghan wrote to her father following their falling out.
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Meghan's letter also detailed her frustrations over Thomas' "untrue" claims that she never helped him financially.
Since then Thomas and Meghan's half-sister Samantha have continued to speak critically of her to United Kingdom tabloids.
The letter, handwritten by his daughter previous year, showed Meghan pleading with Thomas to stop talking to the press.
"I can't tell you how frustrating it is to see that".
The letter was first revealed in an article in People magazine in which five friends of the duchess spoke in her defence.
Meghan laughed as a relative of one of the children showed her a photo on her phone, zooming in on the image to get a closer look.
"While the goal of Meghan's letter was very much to fix the relationship with her father, she knew in her heart of hearts that this was going to be released to the papers", he said.