Learn English – More formal alternative for “get a handle on sth.”

Consider,

put one off [one's] stride

Also, put one off one's stroke (chiefly BrEng)

Interfere with one's progress, distract or disturb one, as in The interruption put her off her stride for a moment, and she took several seconds to resume her train of thought, or The noise of the airplanes overhead put her off her stroke, and she missed the next ball. The first term, first recorded in 1946, alludes to the regular pace of a walker or runner; the variant, first recorded in 1914, alludes to the regular strokes of a rower.

The American Heritage® Dictionary of Idioms

throw one off the track

Fig. to cause one to lose one's place in the sequence of things. The interruption threw me off the track for a moment, but I soon got started again with my presentation. Don't let little things throw you off the track. Concentrate on what you're doing.

McGraw-Hill Dictionary of American Idioms and Phrasal Verbs

make one lose one's train of thought

lose one's train of thought

Fig. to forget what one was talking or thinking about. Excuse me, I lost my train of thought. What was I talking about? Your question made the speaker lose her train of thought.

McGraw-Hill Dictionary of American Idioms and Phrasal Verbs

train of thought

A succession of connected ideas, a path of reasoning, as in You've interrupted my train of thought; now what was I saying? This idiom, which uses train in the sense of "an orderly sequence," was first recorded in 1651, in philosopher Thomas Hobbes's Leviathan.

The American Heritage® Dictionary of Idioms

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