“At first, she brought a lot of joy into my life,” Ms. Haskell said.

But she also took. The women had visited Ms. Haskell’s HSBC Bank branch on Ralph several times, and Ms. Legall had access to Ms. Haskell’s account information. At some point several years ago, she told Ms. Haskell that she had taken money that had been in a savings account that had belonged to Mr. Haskell. She needed it to pay a debt, she said.

“It sounded like maybe she’d give it back to me,” Ms. Haskell said. “I was so naïve.”

The incident had no impact on their bond. “I’m still ‘Mommy,’” Ms. Haskell said. “I saw her a lot.” The two visited nearby restaurants and took selfies that Ms. Legall posted online.

In 2013, Ms. Haskell’s last surviving sister, Marcy, died in Florida. Ms. Legall traveled with her for the funeral. “She helped me on the airplane,” Ms. Haskell recalled. “It was like a little vacation for her.”

Four more years passed this way. Then Ms. Haskell’s son, Lloyd, a physician, received a certified letter that stunned him. A bank was going to foreclose on his mother’s home because she was not paying fees related to a reverse mortgage for $424,000.

Reverse mortgage? His mother lived comfortably within her means. She didn’t travel or buy expensive clothes. A splurge for her was adding to her extensive collection of dolls. What did she need $400,000 for?

He asked her. She said she had taken out the money for Ms. Legall, who needed it to pay another debt. Mr. Haskell, mad at himself for believing Ms. Legall was looking after his mother, went to the police and was referred to financial-crimes detectives. They opened a case and discovered the scope of the fraud and loss.