Erm, one step more....if any of you had actually done the research (including the person in question) this page from the gpl site tells you how to do the copyright protection for the software. http://www.gnu.org/licenses/gpl-howto.html

The copyright included and the distribution of said software to various parties will basically provide enough evidence to prove your case. If all else fails the proper collection of forensic evidence from your machine/backups, emails, and printouts you made can all be used as evidence.