Chapman's wife Beth passed away earlier this year after a two-year battle with throat cancer.
Appearing on "Fox & Friends" with hosts Steve Doocy, Ainsley Earhardt, and Brian Kilmeade, Dr. Mehmet Oz said that Chapman was "running from the truth."
"And many of us metaphorically do that, but he was literally doing it," he said. "And, I learned after flying to Colorado and taking him by his hand and getting into why he was in the doldrums...[why] he was in a lot of it is because of his beloved wife Beth's passing.
"He had a broken heart and people do die from a broken heart, but mostly because they give up hope," he said. "And, she was his North Star."
Oz said that by getting Chapman to realize that knowledge is power and you "can learn things you don't want to know but you can almost always address them," he was able to turn his life around.
He said that Chapman was experiencing "all the signs and symptoms" of a heart attack and that "finally, after numerous efforts, we got the testing, proved what he had, now he's in treatment."
"He has completely changed his life. He used to smoke four packs [of cigarettes] a day -- it started when he was aged 12," he said.
Now, Chapman is down to just one pack a day, Oz noted.