I think it's more that she's marveling at the fact that he's found a way to cure himself of being a complete psychopath more than she's worried that she misdiagnosed him. At the start of the series, he really fit the definition of one. It wasn't until he had Harrison that he genuinely started to change.
As for the doctor herself, I don't know what to make of her. Part of me thinks she may be the actual killer since they're going out of their way not to reveal who it is, and she's the only new significant character on the show, aside from the token black policewoman. So if its not someone we've met, why keep it such a secret from the audience?