Well, if you're arguing with a socialist it's worth pointing out that Marx himself makes a distinction between the left-wing socialism that he advocated and what he called "reactionary socialism," which he saw as right-wing socialism. Fascism wasn't yet a named thing in Marx's time, but for him reactionary socialists were people like Otto von Bismarck and Napoleon III, who were very much the forefathers of "National Socialism." They were anti-Capitalist and anti-Liberal, like the Marxists, but wanted to see a return to feudal institutions rather than the post-Capitalist utopia the left-wing socialists had fantasized.

Worth noting that Marx would also view modern progressives and "Democratic Socialists" as right-wing socialists, but what he called "bourgeois socialists."

So, if your sense of "left-right" (an arbitrary binary) is "socialism left, free enterprise right," then unambiguously fascists are left wing because fascists, at least from a Marxist point of view, are unambiguously socialists.

OTOH, if your sense of "left-right" is something more like "internationalism left, nationalism right" then fascism is unambiguously right wing, but it's right wing socialism.

Yet again, if your sense of "left-right" is based in individual liberty vs. authoritarianism, then whichever side you put individual liberty on, both the fascists and the Marxist-Leninists are on the other side.

Closing note - a good response to "Nazis banned unions" is "so did the Soviets, and for the same reason," i. e. "the workers are represented by the government now, so labor unions, while necessary under Capitalism, are now seditious/counterrevolutionary."