Re: how many back talk would you take?
If he doesn't want an unqualified brown belt on the mat, then he shouldn't award the brown belt.
The face-saving way to avoid doing so would be to tell the student privately, but bluntly, that he is not ready to test. With, perhaps, a list of specific things to work on. That's the way it is handled in most dojos.
The more embarrassing way is to allow the student to test, and fail him. But that's still less embarrassing than allowing an unqualified student to represent the dojo.
Think, for a moment, about the effect of promoting this person. Do you think it will improve his attitude, or worsen it?