Hopefully they just sue him to so that he learns the value of a contract.
The murky legal problem with the GPL and other free software licenses is that there isn't any consideration (no exchange of money), or proof of offer and acceptance.
It's a lot easier to prove with paid software and the clicking of a checkbox.
IANAL, but it would be interesting to know if there's any legal precedents for the enforceability of free/gpl/open source licenses.
The GPL is well-tested in that respect.
My presumption on some of the cases where the GPL has been most successful without going to judgement is that those have been the ones where there is the clearest infringement (i.e., Linksys 'borrowing code' for their routers, etc), i.e., direct copies or only slightly modified versions of the original source.
If it gets far enough, I think it will come down to a layperson's (judge or jury) interpretation (based on lawyer and witness arguments) on whether a theme is like reselling a modified car (i.e., Ruf Turbo - a modified Porsche), or an aftermarket car stereo (i.e., one that reverse engineered an automaker's proprietary connector). Yes, code is intangible, unlike a car, and there are places where my overly simplistic analogy breaks apart, but it's not unheard of to look at it in this manner. A federal judge recently compared mp3 torrent sharing to playing music in a business - not that similar, but in the grand scheme of things, you can see some parallels.