Thomas, 76, has spoken to lawyers and is trying to get the names of the buddies to take legal action.
He also has his take on the American magazine People, which published an article on the five.
Thomas told The Sunday People: “I’m trying to get the names and I’m going to take legal action.
“Lies have been told about me, that’s libel. The magazine could also be held responsible and the people who gave the information – but no one seems to want to give up their name. ”
Meghan, 39, won a High Court lawsuit last month against a newspaper for printing excerpts from her letter.
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Thomas – who had promised to testify against his daughter at a trial – said he handed the letter to the newspaper after he was enraged by an article in the US magazine revealing its existence.
Under the headline ‘The Truth About Meghan’, the article was based on anonymous interviews with the five buddies and was the first public reference to the letter.
Thomas, who lives in Rosarito, Mexico, said in a witness statement that he was forced to take action to defend himself, claiming the article was false and portraying him as dishonest and heartless.
He also filed a letter to Oprah Winfrey asking her to interview him as she did with Meghan.
Thomas said, “This is the truth. I want the truth to be revealed.