"At first I thought she might be his mother," Zahau's former personal trainer, Scott Markey, told ABC News. "They seemed like they were very happy. She was always smiling with him. They had a very good relationship."
Markey, who often saw the two together at Hollywood Fitness in Coronado, said she was "the sweetest girl you could possibly imagine."
"Nothing but good things to say about her. It's horrible for this to happen to anybody, but for her it was a shock to me because she was the nicest girl you could meet," Markey said.
While police investigate, Zahau's sister Mary Zahau-Loehner does not believe that she committed suicide.
The sisters spoke the night before Zahau died, she said, and Zahau talked about her plans for the following day.
"She was going to call mom and dad the next morning and text me throughout the day with updates on Max," said Zahau-Loehner. "She was very hopeful. She had to be strong and there for Jonah."
Zahau and Shacknai seemed happy together, her sister said.
Police said it "could take weeks" for forensics and lab results on Zahau's death, just one element of the lengthy investigation to determine whether her death was a suicide.