Kristen Stewart allegedly cheated on Robert Pattinson on Tuesday (July 17) with another Brit, her Snow White and the Huntsman director, Rupert Sanders.

Us Weekly reports they had a “marathon makeout session.”

“Kristen is absolutely devastated,” a source tells People. “It was a mistake and a complete lapse in judgment. She wasn’t having an affair with Rupert. It was just a fleeting moment that shouldn’t have happened. She never meant to hurt anyone. She’s a good person who just made a bad choice.”

Rupert, 41, is married to British Vogue model Liberty Ross, 33, and they have two children, ages 7 and 5.

Rob, 26, and Kristen, 22, were last spotted together at the 2012 Teen Choice Awards.  They will soon be promoting their upcoming Twilight Saga: Breaking Dawn — Part 2, out November 16.