> Truth? WHO'S truth?

depends on your theory of truth...
sometimes...
there are mind-independent facts...
which make certain thoughts / statements either true or false despite whether anybody appreciates that or not.

(e.g. i have an even number of hairs on my head right NOW)

> It's easy to think there is only one truth, but we know this is not true

do you mean that to be true-for-you
or true for everyone?

(i do love a good paradox)

:-)

> For the example of the person who believes he has no friends, the core difference might be in the definition of a friend.

Ah.

FIRST you have to fix the meaning
(define the term and what is meant by the thought / utterance which resolves any ambiguities...)
THEN you assess for truth / utility...