I think Proverbs 20:6 seems pretty straight forward, but it leaves some room for inference or reading between the lines because it poses a question: but a faithful man who can find?
To me it seems like it is suggesting that reliable friends are extremely rare or don't exist at all. On a long enough timeline, all friendships will waiver at different points, but in the overall long arc of the history of a friendship it's possible to remain committed to success to the end.
It is unclear to me if the writer of this proverb, I assume this is king Solomon, is suggesting that if you fail at any point to be reliable then whole relationship is unreliable or if it's better to find an honest person that admits their faults upfront, but still offers their loyalty.
I am going with the latter.