Well, all's well that ends well I suppose.
Looks like this guy, reddit user throwRAhelp577 has managed to get his cards back and justice has been served on the ex-girlfriend and her cousin.
I can't imagine the feeling of having something that you have loved and cultivated so dearly for so many years suddenly ripped away from you. The collection of Pokemon cards may be valued at $6000 but let's get real here... The time and energy that went into creating that collection is irreplaceable and utterly invaluable.
Here's to hoping that he sees his cards in the next few days and this can finally be put to rest!