I parrot an old Beijer article, and say it's fine for them to have that belief, but they should know it's heterodox among US historians and academics.

The impact of third parties on American politics extends far beyond their capacity to attract votes. Minor parties, historically, have been a source of important policy innovations. Women’s suffrage, the graduated income tax, and the direct election of senators, to name a few, were all issues that third parties espoused first.

John D. Hicks,

Let a third party once demonstrate that votes are to be made by adopting a certain demand, then one of the other parties can be trusted to absorb it. Ultimately, if the demand has merit, it will probably be translated into law or practice by the major party that has taken it up…The chronic supporter of third party tickets need not worry, therefore, when he is told, as he surely will be told, that he is “throwing away his vote.” [A] glance through American history would seem to indicate that his kind of vote is after all probably he most powerful vote that has ever been cast.