Learn English – A word that simultaneously means caring and not caring

Try in the zone

adjective
1. In a mental state of focused concentration on the performance of an activity, in which one dissociates oneself from distracting or irrelevant aspects of one's environment.

Here's another definition

a temporary state of heightened concentration experienced by a performing athlete that enables peak performance (i.e. players in the zone)

I think you are dealing with a real bad case of confirmation bias.

Confirmation bias, also called confirmatory bias or myside bias, is the tendency to search for, interpret, favor, and recall information in a way that confirms one's preexisting beliefs or hypotheses, while giving disproportionately less consideration to alternative possibilities.

That being said this person is clearly unqualified to make OR deny a diagnosis.

References:
https://en.wikipedia.org/wiki/Confirmation_bias
https://en.wikipedia.org/wiki/List_of_cognitive_biases