If you have a line conditioner, does noise matter?

I wouldn't be too sure. My understanding is that only the "power regenerators" such as those by PS Audio actually do the job that your friend needs done. I could be wrong.

Most of the conditioners attemp to "filter" some of the line noise, but not regulate the voltage (except for limiting spikes and surges). Dopogue is correct in that your friend would want a regenerator to control the voltage and truly isolate his system from the grid.

While is doesn't adjust line voltage, a sub category of power conditioner includes an Isolation Transformer.
My PanaMax has all the usual power conditioner claims but also has a 400va iso trans. Into this, goes all my low power and digital equipment.
If I could have found the Furman Elizabeth refs, I may have gone that way. As it turned out, the Panamax was available. Not inexpensive, either.

Many, if not most, amps DO NOT like power conditioners. My experience has been mixed. My Rotel RB-1070 really hated the conditioner. My ancient Carver cube? Seemed good with it.
My PSAudio ICE integrated likes it too, but has its own circuit and Solosist outlet.

When it comes to noise reduction , Shunyada is king .

Next to power regeneration, IMO balanced power is the best bet. The technology is proven to significantly reduce power line noise. I've been using balanced power for the past 6 1/2 years. Unless there are wide fluctuations outside normal voltage range and/or brownouts, IMO voltage regulation is not needed.

I have a PS Audio Power Plant Premier, a DIY parallel AC filter, RC filters inside my preamp and EQ, snubber caps on all tube heaters, ferrites on the heater and B+ lines, and my preamp uses DC for the signal tube heaters. Every single additional noise reduction step decreased background noise and improved the sound, and I started with the PPP as the first one! They are additive.

My understanding of these various line conditioners is that they control spikes and brown outs to keep the volts at 120 +-. Shouldn't this take care of the ac "noise" on the lines allowing only the music to flow through unimpeded?

Conditioning the AC power can be done in 2 ways - line conditioning using passive devices, which is what you are referring to & AC power regeneration which is what units like PS Audio Power Plant Premier & others do.
With a (passive) line conditioner - like you are referring to - no new information can be created. I.E. all that a passive line conditioner can do is to supress the grunge on the AC line. The grunge is reduced but not eliminated. So, some residue grunge still gets into the audio gear. The better your audio equipment & the more trained your ears are, the more likely you are to hear the effects of this grunge (like your friend).
It is very difficult for passive line conditioners to control brown-outs. For example, I have a RGPC 400 Mk1 & it can control brown-outs that last for 1/2 second or less. Anything more, it'll go down as the AC power goes down. All this makes sense - there's only so much charge that an AC power capacitor can hold & supply to power hungry electronics.

re. "the music to flow unimpeded" - this is a trade-off. The more you reduce the grunge on the AC power line, the more stages of filtering you need. More stages of filtering hinders the unimpeded flow of music - all AC power line filtering components have parasitic resistances that do not allow all the current to be delivered to the electronics.
Plus, all AC power line filtering components have their own sonic signature which adversely affects the final sound you hear.

So, to your question:

If you have a line conditioner, does noise matter?

yes, it does.
I feel that it matters less (but it still matters) when you have AC power regeneration.

It would be wacky expensive, but couldn't a motor driven Variac be used to automatically control output voltage to 120, or whatever desired?

03-10-11: Magfan
It would be wacky expensive, but couldn't a motor driven Variac be used to automatically control output voltage to 120, or whatever desired?

Not expensive at all. I have this one (not in my audio system), which I once had occasion to use experimentally. Only $119. Just one problem, though -- it is absurdly noisy every time it decides to run the motor, which was every few minutes at my location.

Best regards,
-- Al