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One of the grammar books I read (I've forgotten which: DEVELOPING GRAMMAR IN CONTEXT, or EXPLORING GRAMMAR IN CONTEXT, both published by Cambridge University Press) says the sentence 'He's unfriendly, isn't he?' is incorrect. It should be ' He's not friendly, is he?'.
My question is: Does a sentence with an un- word never match a tag question with isn't, doesn't, won't, can't, mustn't etc?
It may not be grammatically correct but most American native English speakers (I am one) would understand either. I do not know about "correct grammar" That is a very British concept and not too useful in international English.
He is unfriendly, isn't he? 
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