I know that the software RE-distributor will not read this, but I will list a few advantages and disadvantages of licensing, and the difference between licensing and moral obligations.
1- WP is GPL, and plugins based on "hooking" into GPL products ARE NOT AUTOMATICALLY GPL, as they are not an "addition" to WP code, being incorporated into it, but simply "hooked", and may be licensed in any fashion. Under at least United States law, any code that is developed, is automatically "copyrighted" to the author. It is up to the AUTHOR'S discretion to license at he wishes, and no indication of GPL release was insinuated, so even with readable source, this is NOT a Free as in freedom product (GPL, MOZZ, etc). Just being able to see all of the text in a copyrighted book does not make it "open" to reproduction or alteration in ANY WAY. Not only that, if you wish to enter the code into a commercial product that you wish NOT to have reshared without (or with) than the Mozilla or BSD licensing shemes might be more to your likening than GPL in the end anyway.
2. If the plugin was released under GPL, then anyone who bought it, would THEN have the right to modify and redistribute, but only in the case that they allow others to do the same (such as posting the code with GPL notice on the boards). This is not the case, so please do not share the code here without understanding that it would be a DCMA violation even if you shared the original code.
3. As for the moral issue. BHW has an ethic of allowing shared material, unless a DCMA is filed for DNS. Please add the original file AND the modified (just for safe measure) so that it does not get shared here again without BST interaction, also disallowing BST shares for the modified due to the copying issue.
4. As for refunding money, apologizing, and etc.. good luck. If someone breaks into your home and sells your stereo to someone else, the likelyhood of that person being "moral" enough to pay that someone else back is nill to none. PayPal is prob your best bet, but as it is a digital product, they may not go for a refund at first, until you explain to them that they are selling fake products, just like Likey' tennis shoes from China. He should at least lose his account.
I would suggest he read this and refund, but as he made it perfectly clear how he feels, I myself would just ban him and let those who were ripped off go all out for his PP account (after this, I am sure the money has already been pulled) funds and even shut down.
If you have some time, you can also do a little "reputation management" of your own wherever the person is found to be posting (or search results) on line.