I'll tell you how you can do it. Whether you or your dad wants for you to use the receiver that he's currently using to try this, well...that's up to you two.

It shouldn't hurt anything, though. I did something when I powered two older speakers as subs using an older Yamaha receiver.
The subwoofer that you have contains a woofer cone. That cone should have two wires coming off of it that were/are attached to the amplifier. If you've taken the amplifier out, you've already seen these. Those wires are just the positive and negative leads that will act exactly like the wires that you are used to connecting to speakers (which are normally attached to terminals on the outside of the speaker box).
You would connect the subwoofer output from another receiver (or other audio source) to one of the analog RCA inputs on your dad's receiver. For example, the right channel RCA input for the "CD" input. You would then connect the wires from the subwoofer cone, using extension wire if required, to the corresponding speaker terminal on your dad's receiver (in this example, the front right speaker terminal). You would select the proper input on your dad's receiver (in this example, the "CD" input), turn off any surround sound decoding, start the audio on the source, and adjust the volume on your dad's receiver. I recommend starting with the volume on your dad's receiver at the minimum setting and then adjust upwards.
Does that makes sense?