Literally the only difference between 16 bit and 24 bit is that the latter has a lower noise floor, which is really only useful for sound production - It doesn’t translate to any increase in meaningful detail or dynamic range when dealing with playback.
16-bit was chosen as the defacto standard for CDs and digital music precisely because it contains more than enough dynamic range for human hearing.
Any difference your gf hears is due to the placebo effect rather than any inherent difference in the actual audio.