He has offered the actress the run of his home, in the trendy Sunset Plaza area, until she feels able to head out and face the public again under the harsh glare of her cheating scandal.
"Kristen is acting like a heartbroken teenager," a source tells RadarOnline.com exclusively. "She is crying her eyes out non-stop and does not want to communicate with anyone.
"She is beyond mortified and humiliated and she is also broken hearted," the source says. "She says she truly loved Rob. She says he was her soul mate, but she blew it and now she is scared that she has lost him forever.
"She is pretty much inconsolable."
Stewart hasn't been seen since she issued a public apology to Pattinson, her Twilight costar and real-life boyfriend, for what she called a "momentary indiscretion" with her Snow White and the Huntsman director Rupert Sanders.
"Kristen hasn't showered or changed or washed her hair in several days," the insider reveals. "She is laying around in her T-shirt and shorts and eating ice cream. She really wants to believe that she can win back Rob, but I think deep down she knows that isn't the case."
Agnelli, who produced Stewart's 2010 movie, Welcome to the Rileys, has been tweeting support of his friend, insisting she never had sex with Sanders. "There was no on set affair he wrote last week.
"Giovanni has been very, very attentive to her every need," our source says. "He has been fielding all her calls via her cellphone and vetting who she gets to speak to, so not many people can get through to her right now."