akinks wrote:Is there any way he could use a balanced (or pseudo balanced) connection between guitar & amp to help reduce the interference?

That assumes that most of the interference is being picked up through the cable - you could test this by running a very short cable first I suppose.

To balance/debalance the guitar signal you could use a couple of passive DI boxes and reverse the one on the other end. Not sure it would help much though if you consider what the guitar pickups are doing.

The problem with a faraday cage made from metal screen is that it will only effectively block high frequency stuff (eg, RFI). If you want to block lower frequencies, you need thicker, higher conductivity or higher permeability (this is why high permeability materials like mu metal are used for 50/60Hz). This is very difficult to implement in a large space without really high costs. For sensitive (and expensive) instrumentation, you can use active field canceling systems, but these are quite expensive and not always completely effective.