Differences between cd transports?

Toslink often sounds worse than coax, in spite of immunity to external electrical noise or not creating ground loops.  It is because optical transmitters are slow.  Light is moving fast, of course, but transitions are slow making moment of detecting threshold voltage vulnerable to system noise.  This will produce slight time variations (jitter) resulting in added noise after D/A conversion.  It is possible that your friend's transport had faster optical transmitters (LEDs) or had less system noise (better power supply, better shielding etc).   Coax has much faster transitions (tenfold), but it has other problems like sensitivity to electrical noise, ground loops and reflections in the cable.

@kijanki 
 
Some DACs perform better with Toslink than USB/coax/etc., so a blanket statement about which input is best for a specific DAC can’t be made. 

Assuming transports are working properly, there is also often errors in reading optical discs in real time as CD players must do, especially with poor quality, defective, or damaged discs that can affect results differently from transport to transport.

What Mapman said.
I’ll add the standard being used to transfer the datastream to the DAC; the use of USB, SPDIF coax, AES, or optical. Also the quality of the cable: ie, how well a SPDIF cable faithfully transfers data without adding jitter.

And IME, the design of the transport; I went from using an NAD CDP as a transport to an ARC which was a major upgrade in SQ, both as a CD and as a transport. My quality of playback increased again since adding a PS Audio memory player.

Some DACs perform better with Toslink than USB/coax/etc., so a blanket statement about which input is best for a specific DAC can’t be made.

That's why I said "often". 

I have an Oppo 105 that I was using with a quality WireWorld toslink. I upgraded it’s input IEC and silver wire to the power supply and later I replaced the OEM power supply with a Linear Power Module, but was not happy with the sound, so I bought a Marantz HD CD 1 to use as a transport. When I replaced the toslink with a TOL WireWorld Coax the Marantz took a giant step forward.

I finally got around to hooking my Oppo 105 back up, this time with the Coax. Coax is so much better on these 2 players used as transports. I doubt there is a transport where toslink would sound better. Some newer high end transports no longer include toslink