He should check them first to see that they are in good operating condition (before budgeting amplification). Having them rebuit can cost a lot more than "good" amps for these.
Same as Ivan on the amp (no more than 30 watts and you don't want an amp which can add more than it's rated power as this can arc the panels). Quad designed "current dumping" amps, which were solid state for this application. I recently saw a 303/33/FM3 set (amp/preamp/tuner) for around $600 that would be a classic SS system for the 57's. I owned this rig myself a long time ago, but ran Rogers Ls3/5a's (not Quads) with it. I did prefer the sound of the inexpensive tube amps listed below though still liked the Quad gear (which was easy on the eyes, very small in size and quite an attention getter). The quad preamps also have tone controls that actually work well.
If they are in a smaller room, 14 watts (such as typical EL84 based push/pull tube designs should be enough power and safe to use). Some inexpensive integrated tube amps ($200-$300 in good condition) that fill the 14 watt bill are: Dynaco SCA35, Eico HF81 and Pilot 240 (these are all vintage amps that sound good). A step up if he were to use an amp/preamp combo would be a Pilot 232 power amp ($200-$350 depending on condition). I am using a 232, right now, with Reynaud Twins (they drop to 3.7 ohms) and it is one of the best EL84 based amps that I have yet heard.
If your friend has a larger room then a favorite amp (power amp only I think) used to be one by David Berning which put out 30 watts and was able to run very low (ohm) loads. I can't remember the model number but think that they retailed for around $1K ten years ago, so should be affordable now used. I have never heard a Berning amp, just heard good things about this one from a local Quad user.
The Aleph (or Volks) Class "A" SS power amps (30 watts) are being closed out by some dealer for around $1300 (saw an add recently).
Anyway, lots to chose from to power the Quads if they are in good operating condition.
