He’s wrong, though, anyway. In the digital domain you absolutely CAN affect a single frequency, without adjusting any others, if you wanted to. We don’t, in general, because it’s not very useful and what you almost always want is a graduated ranged of effect around a center frequency.
Regardless, essentially every studio album ever has massive amounts of EQ, and other processing, applied all over the place …
It’s the same with room treatments (bass-traps and so on) though. In most cases they’re actually quite a bit LESS precise than DSP (sometimes grossly so). They’re preferred by a lot of “purist” types simply a) because they’re not in the signal chain and b) sometimes they can address issues that are harder to do in DSP (i.e. would drive up the cost too much).
DSP can address a lot more than just equalization.
At the end of the day, the only way to avoid these issues entirely is to build an acoustically inert room and couple it with a (non-existent) perfect speaker.
Your friend reminds me of Don Quixote.
And this is a very common position/attitude among people that have “very expensive” audio systems. Which is a shame, because expensive is highly relative (his $70K speaker system is less expensive than my primary headphone rig).
At the end of the day, there isn’t much that’s more annoying than someone telling you how to spend your money or what you should, or shouldn’t like. The best way to piss away very large sums of money in audio is to spend any time at all worrying about what anyone else likes.
If you like your music run through a cheap 1980s graphic equalizer, with a massive U-shaped profile, more power to you! That’s no more wrong than it is correct as long as the person writing the check and listening to the music is happy.