As opposed to? All widely spoken languages are derived, and this isn't in dispute. It's not really necessary to prove something that nobody's arguing with. Nobody thinks "The Queen's English" originated with the Queen. I think you'd be hard-pressed to find someone you needed to "prove" your case to. And I think it's because you're looking at the wrong pretentious argument.
Rather, the claim is that only certain forms of "English" should be considered English and that certain sub-cultural groups use of the language is a travesty, etc, blah blah. While it's nonsense that no one form of English should be considered correct, as it is constantly evolving as most languages are, it's also true that once a language derives sufficiently that the majority of speakers English speakers can no longer decipher it, it should clearly no longer be considered "English".
Sure - if the majority of the speakers moved the language that way, fine - that's just the evolution of language. But if a particular cultural sub-group moves their use of the language beyond the point of recognition, nobody should have any hesitation to point out that they can't speak English - because they're not doing so.