It is 49.8% (people who voted Trump in 2024) of 64.1% (people who voted in 2024), or 31.9% or ~1/3 of the total eligible voting population, which is what your parent states.
You understand how statistics work, don't you? When you have 64% of the population voting, that's a pretty big sample size, enough so that you can reasonably extrapolate that the 49.8% share _probably_ holds across the rest of the population, give or take.
Put another way, if someone asked you to estimate what the split between the one third of the population that didn't vote was, what would you use as a reference point? Social media posts? Vibes? Or maybe polls leading up to the vote that showed the same roughly 50-50 split found in the actual results?
That is the problem though - a third of the US population is basically lunatics... This will not go away. And one cannot keep a third of the population "down".