By the same token, the GM's in his rights to say that negotiation in any given circumstance is impossible. It may be that this enemy is simply implaccable, and that there's no common ground for any level of diplomacy to take root in.
That's not to say it's not worth trying. But it's probably not fair to the GM to expect one skill roll to (potentially) derail an entire encounter.
Of course, he may have anticipated that too, and accounted for it, which will lead to a more satisfying conclusion than simply saying, 'it doesn't work.'
Let us see.