Thanks to Frankel, 42, New York City writer Nick Feitel, 25, has had what the Bravo reality star calls "a manformation."
Feitel, who weighed 225 lbs., joined Frankel's website as a culinary columnist (and appeared on her show Bethenny Ever After) last year, though his confidence was at low ebb.
"I wouldn’t go shopping for clothes because I'd be like, Well, these all look bad on me," he says.
"You were sheepish. Self-loathing," recalls Frankel. "I gave him [my book] Naturally Thin and said, You're going to unleash your skinnygirl."
Frankel also introduced Feitel to yoga, which helped him slim down to 159 lbs.
"When I met Nick, he wasn't open, he hadn't blossomed. So I'm just happy that you feel confident and happier," she says. "You've evolved."
Be sure to tune in to the special 20/20 and PEOPLE Half Their Size airing on ABC on Jan. 4 (10 p.m. ET)