I don't see why a low pass filter won't work. The amp has two inputs:
With a low pass filter, all he has to do is bring the signal into the filter, and the filter splits the signal, and then connect the amp from the filter. He would need a sub with a left and right line-level input. Alternatively, some of the SVS subs have built-in 80 Hz high-passed outputs. Furthermore, the computer itself should have some bass management capability. Depending on his computer's sound interface, he should just be able to connect the speaker outputs to the amp, and the sub out to the subwoofer. No filter is needed, no AVR is needed, only a subwoofer. If the computer can not do that, it is better to get a new sound interface for the computer.