I dont think that part was that unrealistic. (SPOILER)People can easily decide to go to great lengths to save someone in a crisis just as people can also go to great lengths to save their own skin and leave someone else behind. We saw both sides in the film. You could tell Rob had an obsession, as Heath pointed out, which is understandable what with all the adrenaline and whatnot pumping into him. The other people, even though they are good friends with the girl, prematurely decide that she is a lost cause and would rather get the hell out. Reality is not all dog-eat-dog imo.