@Mogul

If the AVR has pre-amp outs you can use an analog 6/8ch mixer to do a custom down mix and then feed the output to a wireless headphone transmitter or Bluetooth headphone transmitter. This takes care of the customers needs and makes any input on the AVR available via the mixed output.

If the customer just has issues with the dialog you can use this set up with open back headphones. You can adjust the center dialog independent of the AVR while still letting the surrounds in naturally through the open back headphones.

[Link: amazon.com]