It was reported earlier this week that Timberlake and Biel had decided to marry after four years of dating while on vacation in Wyoming last month.
Gossip Cop quotes Bomar as saying that she was asked to keep quiet about the engagement "until after the holidays".
"He called me on Christmas Day and told me he's engaged," she said.
Bomar additionally told Radar Online: "Jessica is a very sweet girl, she's upright and everything and we love her. She's a wonderful campaigner for the environment and Justin is too, a fine young man. Jessica will be a lovely member of the family."
Timberlake and Biel split in March 2011 after "mutually deciding to part ways".
However, they reunited a few months later and have remained together since.
> Jessica Biel: 'I'm not ready to have children'