R’ Nachman of Breslov taught that when you judge someone favorably, Hashem judges that person the same way that you judged him.
For example, if you see someone eating food that is not kosher, but you think to yourself that he must be eating it for health reasons and has no other choice, Hashem will actually judge that person as if he has health issues and has no choice! How you judge, that is how Hashem will judge!
Can you imagine if we would all just judge people favorably?!
Emuna Daily 
But of Course It’s Only For An Individual That Believes In The 13 Principles.
Your example is a chidush – if it is correct. I always understood this statement to mean if Mr A judges Mr B favorably when he sees him doing something wrong – that the Aybeshtur will judge Mr A favorably – i.e., when the Aybeshter Judges Mr A – he will use the same criteria Mr A used to Judge Mr B favorably. Your example goes further and says that the Aybeshtur will judge Mr B favorably – just as Mr A judged Mr B favorably. Am I correct in my understanding?
Moishe Chaim