Blacks, both pre-Civil War and post-Civil War.
“Capitalists in the Soviet Union” is IMO a bad example because there was nothing democratic about Soviet rule (well, there was a pretense of it – they did hold sham elections in the SovWorld. But if your dad thinks those were real, he is probably a lost cause right from the get go).
Property/Business owners in the United States post-Nebbia are another example but he’s probably not ready for that sort of example.
IMO once he starts carving out exceptions, “oh, this mistreated minority was a religious minority, so it doesn’t count; oh, that mistreated minority was an ethnic minority, so that one doesn’t count either, oh, the other one was wealthy kulaks so since they were prosperous you can hardly count them as ‘oppressed,'” he has already substantively lost. However I am certain there are policies democratic nations have which he does not like, and which can be stacked up as failures of democracy.
One can also point to the fact that in many if not most ways almost all “Western Democracies” are hardly democratic anymore anyhow, with most of the rules we live under being made by permanent bureaucracies.
Probably Hoppe’s book Democracy: The God that Failed can be mined for examples of how democracy fails.
But, really, if he seriously believes that people who disagree with a minority should be forced to comply with it at gunpoint, you’re probably beating your head against a brick wall. HOWEVER, my other way of approaching it is probably thus: such people often have a cognative dissonance, especially if they’re leftists. By which I mean they also tend to believe in “brave progressive dissidents speaking truth to power” and advancing The Movement. You can point out that many (most?) of his heroes were, at the time they were “standing up for social justice,” in the minority on whatever position-of-the-day they held, and by his own theory of absolutist democracy the majority would have had the right to crush them instantly, silence them, and impose their beliefs on them (I wonder if he also thinks he believes “imposing one’s beliefs on others” is wrong, at least when done by people he doesn’t agree with, however numerous they might be). Point out that it is only the protections of liberty – liberty of the minority – however feeble those protections might be, which allowed these “stalwarts” the platform they had.
Probably what he really means though is the right of any temporary majority he agrees with to impose views he shares for once and for all time. Collectivists tend to be like that – this is why “democratic majorities” that protect anti-collectivists policies and positions don’t mean much to collectivists except as obstetrical to be overcome rather than as “the majority has spoken, and we, being sincere majoritarian democrats, will respect and bow to the collective decision we as individuals did not share, out of collective obligation. All praise the wisdom of the collective.” No, they redouble their efforts to undermine and work against it and even use non-democratic institutions to subvert and overturn it and get their own way.
Apologies if your dad is not like that but is a sincere collectivist majoritarian who hops on whatever bandwagon is most popular at any given moment, be it the Iraq War ten years ago or opposition to Obamacare (opposed by a majority of Americans both when passed and still) now.