Laura Griffin was Liam's high school sweetheart for six years until Liam broke it off to be with Miley in 2009. The actors got engaged earlier this month, which Laura found surprising.
"Hearing your high school sweetheart, who is the same age as you, is getting married, is a shock," the 22-year-old Melbournite told Celebuzz.
"Of course it hurts seeing someone I spent so many years with moving on … But at the same time if they are in love and are happy why not? I'm very happy for him."
Liam and Miley met on the set of The Last Song while Laura and Liam were still dating, but they broke up before Miley starting dating the 22-year-old actor.
"We were together when he left for America to film The Last Song and we broke up after a few months … Liam never cheated on me, that was portrayed in the media, but he had ended things with me when Miley and him began dating," Laura said.
"We were ready for different paths, America was his future and Australia was where I needed to be ... I'm his past and I've accepted that."
As shocking as she finds it, Laura is defending her former beau's decision to get hitched at such a young age.
"If that person is your person and the world seems perfect with them in it, then yes, I do believe it can work … I definitely think a successful marriage is possible at this age," she said.
Today Miley slammed rumours that she's cheating on Liam with friend Cheyne Thomas, after the pair were spotted in a swimming pool together.
"I love my fiancé and he loves me. All the rest is bulls---," she wrote on her Twitter account.
"So now because I am engaged I can't have a friend of the opposite sex? Can't have a friend help me out while I’m working alllll [sic] day? #dumb."
As for Laura? She wishes the couple "nothing more or less than congratulations".
"He would know I meant it," she says.