Robert is said to be dating Elvis Presley's actress granddaughter Riley - who starred with Kristen in 2010 film 'The Runaways' - after the pair were spotted at his home in Los Angeles last month.
Kristen split from Rob, 27, for the second time in May, after he discovered she had been back in contact with her 'Snow White And The Huntsman' director Rupert Sanders who she had an affair with behind Rob's back last year.
The 23-year-old actress is apparently unhappy with her former lover's new romance with her friend and has been bombarding the pair with messages and telephone calls.
A source told the new UK issue of Grazia magazine: "Kristen is furious and has texted Rob and called Riley to find out what is going on, but apparently none of her calls have been answered."
Robert is believed to have first met Riley through Kristen.
Things are said to be hotting up between the new couple, as they are planning on spending the summer together.
The source added: "Rob only knows Riley because of Kristen. Now they've been inseparable in the last few weeks and have been having low key dates at his LA home.
"Kristen is seething. She understands things are over between her and Rob, but to see him now dating one of her closest friends is just too much for her to deal with and seeing the pictures really hurt her."
Riley's rep has denied the pair are dating.