Reducing one frequency in song
Zeroing out bins / attenuating them in discrete Fourier domain is universally a bad idea, due to the undesirable time-domain effects of that.

Instead, use a audio processing program to apply an adjustable equalizer to the song of choice. Let's walk you through Free software:


Get audacity;
load your song in that, open "Effects"->"Equalizer"
start with one  of the presets to see how things work, and then
sit down with your grandpa and figure out what frequency ranges to dampen.
Save that preset!
You can later apply it to all your music collection using batch processing.


You should also, in that Equalizer dialogue, "manage / export" your freshly designed filter frequency response. With a little work, you could, for example, build a digital equalizer (in music, your granddad probably won't mind the latency that brings, which would be a bad thing in live music) using e.g. a Raspberry Pi and a USB soundcard, which you could put in between you granddad's CD player and his amplifier, or so. (of course, it could also act as media player/server/center/whatevs.)