If you want to find out where the warmer sound comes from, connect the turntable to the piece with a phono preamp, connect a stereo cable from its Rec Out to the Aux or Tape In of the receiver and play an album. If it sounds the way he wants and the level is adequate, leave it, but show him how to operate it RELIABLY and leave a cheat sheet- he will forget how to do it.

If it sounds good and he likes it, get a phono preamp with impedance and capacitance compensation. Connect the Rec Out from the piece with the phono preamp to the Sonos- connecting the output of a phono preamp to two pieces of equipment doesn't always work well because it's not necessarily designed to see that load.