If it really is the case that this is copyright infringement, the easiest step may be to send a DMCA complaint to GitHub. The purpose of the DMCA complaint mechanism was to make it cheap and easy to deal with online copyright infringement without resorting to the courts. I know we're used to hearing about silly and illegitimate DMCA complaints, but this would seem like a reasonable use of it.

(This isn't legal advice, of course; I don't know enough of the facts to be sure what applies.)