The 27-year-old British actor dumped his former 'Twilight Saga' co-star - who had a brief fling with married director Rupert Sanders last summer - recently after more than four years together because his friends, including the 'Firework' singer, reportedly told him that he deserves better.
A source told Life & Style magazine: "Katy never really supported Rob's decision to get back with Kristen. She always said they wouldn't last.
"Katy told Rob she didn't think their relationship could withstand the cheating scandal. And he deserved better than someone who would destroy a family [like Rupert's]."
Despite the couple managing to patch things up after a two-month break following Kristen's "momentary indiscretion" with Rupert last year, it is thought their relationship is over for good this time after Rob was spotted moving his belongings out of her home in Los Feliz last on Sunday May 19, where they both lived over the last few months.
A large moving truck picked up the last of his belongings the following Thursday.
A source previously said: "After everything that happened and the hurt she caused, he's furious she'd be back in touch with him, even if she is saying it's completely innocent.
"It does look like this it's over for good. He simply can't trust her."