Harris jetted off to Las Vegas for a pool party at the city's Wet Republic mere days after splitting from Hefner and moving out of the Playboy mansion.
The 25-year-old was seen with the likes of Hills star Heidi Montag at the bash, her first public appearance since the break-up.
"I just knew when it was getting close [to the wedding] it wasn't fair to [Hefner] and wasn't fair to go through with it," she told reporters at the party. "I wanted to be true to myself and be true to what was best for everybody.
"I love [Hefner]. He is the sweetest person I've ever met in my entire life, and I'll always love him and he'll always be a dear friend and I'll always visit him all the time."
However, Harris's sentiments were not accepted by the 85-year-old's ex-partner and occassional Playboy model Madison, who argued that she had behaved 'disgustingly' to Hefner.
"That's a new low," Madison wrote on Twitter. "That's disgusting and whoever booked her is tacky."
Hefner has welcomed a pair of twins he used to date back to the Playboy mansion in the wake of the split. He is also alleged to have struck up a romance with the magazine's Miss January Anna Sophia Berglund.
Madison confirmed the end of her relationship with Hefner back in October 2008.