He was 59 and had cancer of the esophagus, the Associated Press reported.
Kardashian died at home Tuesday, another friend said.
"It's shocking when a friend close to you passes," Simpson said. "I loved Bobby."
"We had one disagreement over the years, about a book he did for money," he said. "He explained it to me, I understood it, and we put it aside."
Simpson added, "Bob was there when I needed him most."
When Simpson failed to turn himself in to authorities to face murder charges on June 17, 1994, it was Kardashian who went before TV cameras and read an emotional letter from his long-time friend.