The Welsh star, who dated TV presenter Steve from 1999 to 2002, discussed the betrayal on an episode of Dish podcast with Nick Grimshaw and Angela Hartnett.
Alex was working as an entertainment correspondent for the Channel 4 show Rise when she was given the opportunity to interview the Oscar-winning actress.
The One Show host revealed that Steve had told her bosses that she was unwell when she was actually just out shopping.
"Normally, they would call in the morning and ask to come and do an interview or a junket," the MailOnline reports Alex's interview on the podcast.
"I was staying in his flat at the time when I was working in London and he picked up the landline and said 'Al's not here. She's not feeling well, she's had to go back to Cardiff', I was in Topshop," she explained, adding: "So he did the interview."
This isn't the first time that Alex has told this story. While appearing on the Thanks A Million podcast with Angela Scanlon, she went into more detail without naming Steve. "I got back to the flat and was like, 'where is he?'" she said. "Hours went past, nobody answering their phones and in the end thought, 'Right I'm going to drive back to Cardiff, because I don't know where you are, you've probably gone to the pub.'
"And then the production company called, and they were like, we've got bit of a situation. Tomorrow morning, your boyfriend is going to the premiere with Angelina Jolie and it it would be really funny if you came on the line, live on the show and surprised him and said that you know exactly what he’s been up to."
Alex went on to say that she accepted the offer, and the pair "split up live on telly".