How to describe someone who can disregard their own knowledge and experience

Solution 1:

You describe the ability to step outside yourself or to be dispassionate.

Definition of step outside yourself:

To try and see things from another perspective, as if you were someone else looking at your life and situation.
Using the expression Step Outside Yourself, from Quora, Aug 23, 2016

Definition of dispassionate:

Not influenced by strong feelings or emotions; impartial: a dispassionate reporter.
American Heritage Dictionary, 5th Ed.

Solution 2:

I think objective would be an appropriate descriptor.

They are a very objective person.

My friend is completly objective, even though he is very opinionated and knowledgeable on most subjects he can dispassionately consider and discuss both sides of an issue.

objective |əbˈjektiv|

adjective

merrium webster:

3 a : expressing or dealing with facts or conditions as perceived without distortion by personal feelings, prejudices, or interpretations objective art an objective history of the war an objective judgment

Apple Dictionary app

1 (of a person or their judgment) not influenced by personal feelings or opinions in considering and representing facts: historians try to be objective and impartial. Contrasted with subjective. • not dependent on the mind for existence; actual: a matter of objective fact.

Solution 3:

For the user manual, such a person has the ability to put himself in a layman's shoes:

put oneself in someone's shoes (idiomatic): To try to look at a situation from a different point of view; as if one were the other person. To empathise. (wiktionary)

For the devil's advocate or judge, this person has the ability to sum up both sides of an issue. Furthermore, he is articulate.

sum (something) up: to give a summary of something. I would like to sum this lecture up by listing the main points I have covered. It is time for me to sum up. She summed up the president's speech in three sentences. (thefreedictionary.com idioms)

articulate (adj.): expressing or able to express things easily and clearly. The First Lady was an intelligent and articulate spokeswoman for a lot of causes. (Cambridge)