Is there anyway to ruin a sub, mechanically that is?

Not necessarily from exposure, but a sub can definitely be ruined from internal damage. If the woofer was dropped hard, it could lock the voice coil assembly or at least severely affect its alignment. Or, a manufacturing defect could be to blame. Broken wire somewhere, could be anything. Test the voice coils by connecting a digital multimeter and see what the impedance (ohms) is showing. This can help you troubleshoot.

A sub that has been "blown" can also show no physical signs of damage, yet not work. Try pushing the woofer cone in and out. You can "feel" a kind of rough, gritty feeling if the voice coils are burnt/damaged.

Hope this helps, perhaps you can describe anything you might find out of the ordinary.

most likely it is your voice coil, also i would connect the sub that works to the other channel (if on sepereate channels) of the amp to be sure its not the channel of the amp ur using, and also stated above blown subs, push down in middle of sub and if its blown you will hear a faint grind sound

You can also hook up your subs to a 9volt battery (drill batt.) and touch/hold the + wire to the + batt. and then tough the - side of the batt. and see if it jumps. That's what some shops do to check if speakers are still good.

Doesnt sound good, never heard of it happening, but I dont doubt that it could happen. Hope he bought them at a good place so he can return them...