Dahl, 52, was found dead in her home Friday, apparently having passed away in her sleep, The Denver Post reported.
"She died of a broken heart and probably the stress but you know you never would have known it unless you knew her really, really, really deeply," said friend Jewel Wellborn.
After losing her husband, Dahl became a public face for those who lost loved ones in the Sept. 11 attacks. She started a scholarship fund in her husband's name that KUSA-TV, Denver, reported has given out 50 scholarships in the last 10 years.
Wellborn said Dahl truly believed her husband, Jason, fought alongside passengers to keep hijackers from crashing the plane into the U.S. Capitol, crashing instead in a field in Shanksville, Pa.
"They did what would almost never be asked of anyone," Dahl told the Post a year ago. "I want to make sure history is written."
"She went out to Shanksville and made sure that everyone had a true memorial portion," Wellborn said. "It was important to her."
Underneath it all, Dahl suffered nightmares and had trouble sleeping. She had been diagnosed with post-traumatic stress disorder.
Family friend Tom Bush said Dahl was otherwise in good health, although the cause of her death has yet to be determined, the newspaper said.
"She was a brave woman, I think a soldier herself in many ways, standing up for this country just like her husband did," Bush said. "It's kind of appropriate that it's Memorial Day Weekend."