high low adapter

Yes he can.

I dont really think it matters. They are mostly made for car audio applications, so that is often the best place to get them. Here is one: http://www.parts-express.com/pe/showdetl.cfm?partnumber=266-002

I have never used one, but I imagine there would be some loss of quality, just due to amplifying and reducing level, then re-amplifying again.

Although it's possible, it's not generally recommended. A return question to ask him though is why he thinks he needs to add an external amp? Is his current receiver volume maxed out and causing the speakers to distort? If not, he likely doesn't need the additional amplification.

The worst thing about the adapter isn't the adapter, but the fact that the signal goes through a power amp. IME experience a good power amp such as is found in AVRs takes from 5 to 10 passes through the power amp to be detectible. This adaptor adds just one pass so there shouldn't be an audible problem. I have some very picky friends who use these in some very large great-sounding systems.

When I asked some audio engineers about this solution years ago, their reply was that it turned the receiver amplifier in to a voltage amp and that the receiver amp would be loafing and very happy.

BTW, once upon a time, such an accessory was offered by high end home audio companies Carver and Soundcraftsmen.

Jeff