According to the New York Post, Pattinson, 26, was "stunned" when Stewart, 22, revealed that she was captured on camera making out with Sanders, 41.
So stunned that he's now left the Los Angeles home they share and is no longer in contact with her, reports People.
"I'm not sure they'll be able to recover from this," concedes a source, who describes the British thespian as "heartbroken and angry" over the betrayal.
Word is, K.Stew confessed all to R.Pattz when she learned the photos were going public.
On Wednesday, the same day Us Weekly hit stands with photographic proof of Stewart's "steamy rendezvous" with Sanders, she released a statement apologizing for the "momentary indiscretion" that "jeopardized the most important thing in my life, the person I love and respect the most, Rob."
Pattinson's apparent lack of communication might explain Stewart's impassioned and nearly unprecedented mea culpa, which came three years into a relationship that neither party had actually confirmed to the general public.
"Rob is in shock that Kristen cheated on him," an insider adds to Radar Online. "He is absolutely devastated. But right now, he doesn't want to say anything in public until he figures out what he's going to do."
Maintaining a wall of silence could prove challenging in the coming weeks as he hits the promotional circuit for David Cronenberg's "Cosmopolis." Then again, he and Kristen apparently weren't keeping things too quiet in the days leading up to the fling revelation.
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