In a program scheduled to air July 24 on British television, Hewitt also said he believed Prince Charles knew about Diana's extramarital affair with him.
"I think he would have been told by the security forces and other people who make it their duty to know what other people are doing," Hewitt said. "I think he would probably be grateful someone was looking after his wife when he was (involved with) Camilla Parker Bowles, don't you?"
The documentary, "Confessions of a Cad," shows Hewitt seeking a buyer for his love letters from Diana.
Hewitt's lawyer said last year that the former army officer planned to sell the letters, but only on condition that they never be published.
Hewitt has been the frequent object of derision in the British press since he cooperated in a 1994 book about his relationship with Diana, called "Princess in Love." She acknowledged the relationship and said he had let her down by speaking publicly about it.
The affair began when he gave her riding lessons and the two are believed to have exchanged many letters, handwritten on blue airmail paper, when he served as a tank commander in the 1991 Persian Gulf War.
Asked in the program how many women he's been involved with, Hewitt answers "This week? This year?"
"I never wanted to be a cad, but I guess I am, so, if you've been handed a bunch of lemons, make some lemon juice," he said.
Hewitt's lawyer and friend Michael Coleman reads during the documentary from what is said to be one of Diana's letters to Hewitt while he was in the Gulf.
"Boy, oh boy, does the earth shake when I get a letter from my desert friend, screams of delight, tears, you name it," Coleman quotes her as writing. "Demented female on the loose, that's for sure."