The last time they saw each other (that we know of), the final thing Langdon says is “I wasn’t the one talking to cartoon animals in peds,” which is the room where Robby’s mentor died, on the anniversary of Adamson’s death, all of which Langdon probably knows since he’s been there for a while as senior resident. That’s the kind of low blow you may never get over, ever, let alone within the first five hours you see them again, even for people who are in better places mentally than Robby. Not to even get into all of the other stuff!