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.

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". 

As long as data from CD is bit perfect the only thing that affects the sound is jitter.  Jitter, having many different forms (correlated, uncorrelated etc.), can produce many sound signatures.  D/A converter word clock's time variation (jitter) converts into many additional (very low level) frequencies on the analog side - basically a noise. Jitter can be produced by transport, by connecting cable or by the DAC itself.  DACs have ability to suppress jitter and all of them do, either by buffering combined with PLL or by Asynchronous Rate Converter (resampling data to D/A convert with new stable clock).