With regard to vinyl vs. CD - it all depends on how the music was produced. The CD, being 100% digital, allows you to encode anything - any sound that fits the dynamic range. The vinyl, being limited to the physical track, allows you to encode some sounds, but not the other. During production these limits were known and taken care of.
So if someone is intent on producing a super-wideband CD (with 41 kSa/s you can encode up to 20.5 kHz) he can do it. If someone wants to record the same 20+ kHz onto the vinyl, that might be not that easy.
The dynamic range of the CD is also larger. If you have 16-bit samples (which are the standard today) this yields 96 dB. This is a huge range. The dynamic range of vinyl is about 80 dB.
So all things considered, IMO you should take all the vinyl that you have and encode it (without lossy compression!) into the digital format. Then you can make backups, listen to the music wherever and whenever you want, and the original media will not be worn or scratched.
With regard to your friend's opinion, the transients are defined by the frequency response of the channel, and the frequency range of the CD is limited only by the filter after the DAC. You can build as good a filter as you want, just throw OAs at the problem. Expensive audio cards (FireWire or USB) already have those filters done right. The card in your PC most likely has abysmal performance, since they can't afford too many components.
With regard to "more complete harmonics" I don't know what it means. A harmonic is really an unwanted artifact produced by nonlinearity of something in the channel. Harmonics do not exist at the source, they are added by the machine that recorded the music and played it back. In reality both the instrument and the ear are highly nonlinear, and there are lots of harmonics produced naturally, but here we want to evaluate an ideal channel.
All in all, a modern digital recording and playback system is far better, technically, than the vinyl. The main difference is in what is recorded.
Vinyl does indeed wear out.
But I am not sure recording it to standard CD wav format really is better. It’s only 23KHZ, I know, people will say that unless you’re some kind of bat you can’t hear the difference, but I swear it sounds different to me!!
I kbow for a while back in the 80’s I transferred a bunch of CD’s to R2R (I had a nice Sony R2R that would go to 7 1/2 ips) and playing it back from tape sounded better than the original CD’s!