(I'm not making claims for my tube amps beyond my subjective impressions, but just accepting this "tube amp effect" for the sake of this discussion...)
IMHO, your friend's system neither added nor removed information from the actual source file. His/her system decoded and reproduced information as closest to the source file as possible regardless how good or bad subjectively (based on your subjective preferences). I bet if his/her system is fed with exemplary mastered file, it would've had the same emotional/illusionary realism that is heard in real life. Unfortunately, most music mastered today is far from being natural sounding (subjectively) especially through a transparent system. Vinyl currently fares much better in this regard than digital. This is where pairing a tube amp (with desired harmonic distortion characteristics) would take off the "digital/electronic glaze" to make a poor recording more tolerable sounding hence it triggers a listener's emotions as more engaging than a more transparent system.
I think there is certainly some intuitive sense to much of that.
Though my experience is this: I have listened to excellent, very natural recordings on his system and then mine. Those recordings certainly do sound fairly startling on his...but also on mine, and in some ways I still prefer what I hear on mine. So for instance, a well recorded drum track with the drummer playing various parts of the snare, hitting rim shots as well, playing around the high hat. On his system the transients will have great precision, clarity and acoustic force. And the clarity and lack of distortion means that each hit is that much more individual - the rim shot that much more "hard/metal" sounding than the snare hit, the high hat that much more "metallic" than on my system. In those parameters I hear the sound as "more like the real thing" on his system. But on mine, the sound is still less "squeezed" sounding, seems richer and more full - an aspect I hear in real sound - and seems to "breath in acoustic space" more convincingly. So in that respect it sounds "more like the real thing to me." It depends what I cue in on. And I think such an evaluation will also tend to depend on what the individual listener cues in on as the "more realistic/natural cues" in the sound. I've just found that the tube aspects are the ones I miss most when I use solid state.
On a similar note: I owned MBL radialstrahler omnis for many years. They are brutally inefficient and I original tried some solid state amplification (e.g. Bryston). The thing with the MBLs is that, being an omni-directional speaker, they radiate sound differently in to the room, engage the room acoustics, and so have a more effortlessly 3 dimensional sense of space and imagine. They truly were spooky to listen to, like musicians were just transported down in between the speakers. No other speakers I've had (or heard) did this aspect quite like the MBLs. They were also, especially with the bryston amp, incredibly detailed, like my friend's system.
But, crazy me, I happened to have an old Eico HF 81 14w/side integrated tube amp at the time and threw it on the MBLs. Well waddya know! Yeah the bass got a bit more wooly and sloppy (though not too bad) but for me the sound just got "better" in almost all the ways I care about.
Joseph Audio has a statement on their website:
"Live, unamplified music has unmistakable presence and clarity. Yet, at the same time it also sounds relaxed and warm."
That really captures the gestalt, the general impression, I have of live unamplified music as well. And with the tubes on the MBL they just became more of that: retained amazing clarity and vividness, but it became more "relaxed and warm" and filled out and even more "natural and organic" sounding to my ears.
Having had these experiences so many times, while certainly not scientific, dialing it in to my taste has resulted in finding my system(s) being far more satisfying relative to what I usually hear elsewhere.
Whereas someone else may say
"Look, I'm not comparing the sound to live anything, I just want to hear what's on the recording, and so I can put together a system by looking at measurements, and once that is satisfied I just accept the sound as it is." That will of course work for people with that goal as well. But not everyone comes to the hobby with exactly the same experiences, goals, criteria etc.