Kristen Stewart was said to be left mortified when she realised she was standing near Liberty Ross at the Coachella music festival.
The Twilight actress, who was pictured kissing Liberty's former director husband Rupert Sanders, was spotted looking "pretty upset" and "uncomfortable" as she attended the Jurassic 5 gig with her on-off boyfriend Robert Pattinson.
"Kristen has tried so hard to put everything from last year behind her – and she still feels incredibly guilty," an onlooker was quoted as saying.
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"She and Rob had turned a massive corner in recent months and they were both there to enjoy themselves.
"The last thing she wanted to do was make Liberty feel uncomfortable, either. She looked pretty upset in the backstage bar later on that evening."
Model Liberty filed for divorce from Rupert, her husband of 10 years and father of her two children, in January after his "momentary indiscretion" with Kristen.
The couple's relationship was under immense strain after news broke in July that Rupert had conducted a brief affair with Kristen, who was also in a relationship with Robert Pattinson at the time. The Twilight actress swiftly issued a heartfelt apology for the "momentary indiscretion" that occurred when she was working on Rupert's film Snow White And The Huntsman, which also starred his wife Liberty.
Her statement was quickly followed by a similar apology from 42-year-old Rupert to Vogue model Liberty, saying he was "utterly distraught about the pain" he had caused his family.
"My beautiful wife and heavenly children are all I have in this world" he added. "I love them with all my heart. I am praying that we can get through this together.'
Kristen's statement read: "I'm deeply sorry for the hurt and embarrassment I've caused to those close to me and everyone this has affected. This momentary indiscretion has jeopardized the most important thing in my life, the person I love and respect the most, Rob. I love him, I love him, I'm so sorry."