Sarah Matulin, who met the Victoria Cross recipient while they both worked for media giant Seven West, took aim at Mr Hastie in a comment on Instagram.
Ms Matulin wrote "yeah you're a traitor" underneath a post of Mr Hastie and his children on Anzac Day, which had a caption about the importance of remembering fallen soldiers.
Her comment was later deleted.
In a statement through Roberts-Smith's lawyer, Ms Matulin conceded the online remarks were an error.
"It was a mistake to make that comment in a public forum and wasn't done with Ben's knowledge," she said.
Asked about the comment, Mr Hastie struck a philosophical tone.Â
"It is what is is, and I really have nothing to add to it. Life goes on," he told Sky News on Tuesday.
"I'm just very cautious, given that a fair trial, the presumption of innocence and a few other legal principles are at stake here, so I won't be commentating like other public figures on this," Mr Hastie said.
When Roberts-Smith sued for defamation over a series of newspaper articles accusing him of war crimes, Mr Hastie was called as a witness by Nine Entertainment, the publisher defending the suit.
The now-Liberal MP was a captain in the SAS for more than five years and was deployed to Afghanistan alongside Roberts-Smith.
Mr Hastie said he may be called as a witness in the upcoming criminal trial.
Roberts-Smith has been charged with five counts of war crime murder over a series of alleged killings in Afghanistan.
