Randy and Amy Stoen were invited to appear on "The Oprah Winfrey Show" to discuss their 3-year-old daughter Madelaine's diagnosis of and treatment for alveolar rhabdomyosarcoma, a rare, aggressive cancer that has attacked the left side of her face, the Star Tribune, a Minneapolis newspaper, reported.

The show found out about the Stoens' situation when a family friend sent an e-mail.

"They really expected nothing other than that they might be instrumental in helping some other family," Elaine Schmeling, one of Madelaine's grandmothers, told the newspaper.

But Cowell surprised the Stoens by telling them he paid off their mortgage.

"I didn't know what to do in this situation other than to do something which I hope will help," Cowell said during a segment on the "Oprah" episode that aired Monday.

"I know that you're having problems with your mortgage. And as of this afternoon, your mortgage has been paid off," he said. "If there's any problems, you know, I'm a guardian angel now."

Amy Stoen called Cowell's gift an "overwhelming relief," adding, "I don't know if it really has hit me yet."