Kristen, 23, who split from Robert, 27, in May, is said to be extremely hurt he has not wasted any time in finding a new girlfriend.
A source told the Daily Mirror newspaper: "Kristen feels betrayed and hurt that Rob is moving on so quickly after their intense four year romance."
Robert - who was also linked to Katy Perry after being spotted with her at a Bjork gig in May - is even thought to have first met Riley through Kristen.
Things are also said to be hotting up between the new couple, as they are planning on spending all their time together before Rob jets off to Toronto on Monday to start work on his new film.
A friend added: "Riley and Rob are glued to each other's side. Rob's awestruck that he's dating someone related to Elvis as he's a massive fan of rock 'n' roll."
If their relationship blossoms, Robert and Riley could even potentially star in a join advertising campaign. Robert is the face of aftershave Dior Homme, while Riley used to represent perfume Dior Cherie and still has ties to the company