Jun 8, 2011 09:55 GMT  ·  By

Jennifer Aniston and her rumored beau Justin Theroux were spotted together at the MTV Movie Awards 2011, thus confirming week-old reports that they were an item. As it turns out, back when they started dating, Justin was not single, it has emerged.

The actor, writer and director has been in a solid relationship with stylist Heidi Bivens when he started working with Aniston on “Wanderlust,” the film where they first became acquainted.

Even more, Star magazine writes in its latest print issue (story via here), when Justin and Jennifer were already dating, Heidi had no idea that she was no longer with Justin.

“On May 16, the fashion website refinery29 posted an interview with NY City based stylist Heidi Bivens, who had raved about the vacation she had taken last winter in Kauai, Hawaii, with her boyfriend, actor / writer / director Justin Theroux,” Star writes.

Two days after the story came out, Justin was out and about with Jennifer, while Heidi still believed she was Justin’s girlfriend, the tab claims.

In other words, when she became romantically involved with Theroux, Jennifer must have known that he was in a relationship – yet she agreed to have him cheat on his girlfriend with herself, despite being once at the receiving end and knowing what it feels like.

“[Heidi is] heartbroken and doesn’t know for sure what’s going on. And the most shocking thing is, Jen definitely knew about Heidi,” a friend of the 34-year-old stylist says for the magazine.

“Even if Justin had been telling Jen he was suddenly single, you’d think she would be 100 percent sure before cutting in on another woman’s man, after what happened to her with Brad Pitt and Angelina Jolie. That is so sad…” adds the spy.

Star tried to reach out to Heidi for comment, telling her that Justin and Jennifer had made their first public appearance as a couple, to which she replied, “I don’t want to know” before adding, “I have no comment.”