Convincing someone that the conclusions of mainstream scientists are generally trustworthy when they are wielding some sort of conspiracy-laden argument against them isn't easy.
Note: I don't use the term 'alternative scientists', rather preferring the much more accurate 'pseudoscientists' (false scientists.) Calling them 'alternative scientists' presumes two things: First that they are scientists (they are not) and second, that mainstream science does not use alternative methods to forward science (it does, but when they work, those methods become mainstream.) And, as a result, the word I use for 'mainstream scientists' is just 'scientists'.
Four possible approaches (mix or match for best effect):
(1) Easy Button: Have them watch videos on YouTube on the subject 'pseudoscience'. This illustrates the other points I make below.
(2) Attack the assumptions: Science isn't controlled by the elite, it is controlled by what works. And nobody prevents pseudoscientists from spreading their ideas, but you don't get into journals for being flatly wrong. Their ideas are rightly rejected, to the point where most pseudoscientists don't even subject their ideas to peer review, instead opting to write books for profit.
(3) Demonstrate the difference between science and pseudoscience: Science starts with observation, forms a hypothesis, tests it, and if the tests work, comes to a conclusion, publishes, has the testing repeated by others, finally forming a rigorously tested valid conclusion. Pseudoscience starts with observation, comes to a conclusion, includes evidence that supports it and ignores any conflicting data. Which of these is more likely to work?
(4) Observe that science works: Science forms the backbone for all the cool tech that works. Computers? Science. Medical technology? Science. Space shuttle? Old, but still science. By contrast, pseudoscience makes nothing, because its conclusions are sloppy and/or outright inaccurate, and you can't make anything from it that works.
