The couple's relationship was publicly revealed on Wednesday when photos surfaced of them holding hands in a car during a vacation in upstate New York and cozying up at a bar. They both shut down their Instagram accounts hours later.
The source says that there was no overlap between the Good Morning America co-anchors' romance and their respective marriages.
"This was two consenting adults who were each separated. They both broke up with their spouses in August within weeks of each other," the insider adds. "The relationship didn't start until after that."
Robach, 49, is married to actor Andrew Shue while Holmes, 45, is married to attorney Marilee Fiebig. Both couples were married in 2010.
Continues the source: "She's got nothing to hide. They were both separated so they felt very comfortable dating in the open after that. Their spouses had moved out, even! So they were not hiding anything."
The insider also tells PEOPLE Robach plans to be on the air Thursday morning.
Another source previously told PEOPLE that Holmes and Robach's GMA colleagues were somewhat surprised to see the new photos, though the pair's chemistry had been on display in the newsroom for a while.
"There were rumors they were having an affair about a year ago," that source said. "A lot of people believed there might have been some truth to it, because you can see there's a mutual affection there. But everyone ultimately chalked it up to friendship because they always said they were both happily married."
Holmes joined the GMA team in 2014 and immediately struck up a close connection with Robach. She previously told PEOPLE about their friendship. "[They] said, 'Hey, we want to bring on a co-anchor. What do you think of T.J. Holmes?' And I almost fell out of my chair," she said.
"My God. You couldn't have picked a better person to ask me what I thought of, because we've been trying to figure out a way to work together for literally the last five years."
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Robach also told PEOPLE about their double dates with Shue and Fiebig. "The moment he started at ABC, I think we just clicked. We've gone on tons of double dates with our spouses and my daughters babysit his daughters."
The recent photos aren't the first time Robach and Holmes have been spotted together off screen. Back in April 2021, Robach shared a photo of herself and Holmes on the streets of New York during one of their marathon training sessions.