I do love the idea of voter registration oscillating back and fourth at 20 minutes intervals forever. Would make voting in the primaries way more exciting as the voter base kept flipping.
Or, in effect, are you just required to claim either that you're more of a cat person, or that you're more of a dog person?
The last time anyone tried to poison a presidential election by promoting a weaker candidate on the other side in the US, it was the Democrats boosting Trump in 2016. It did not work out.
If they want their own private primaries, then it should happen internally and at the parties' own expense.
The thing is that that one plays on propaganda that people have already been conditioned to accept.
Very probably this person's father believes that the Democrats (a) control the state-operated voter registration system, and (b) manipulate it to their advantage. He believes that because he's been sent that message through a vast number of channels for many years. He would think it was absolutely in character for his registered party to be changed, and would probably think that would somehow affect how his vote was actually counted.
It's no more absurd than the idea that busloads of illegal aliens are showing up to vote "somewhere". Or whatever other idiotic lies they've been telling forever.