If you released it under the GPL, then they would not legally be allowed to make a proprietary version and start selling it. Also, the GPL does not prevent you from selling your software: https://www.gnu.org/philosophy/selling.html
If you released it under the GPL, then they would not legally be allowed to make a proprietary version and start selling it. Also, the GPL does not prevent you from selling your software: https://www.gnu.org/philosophy/selling.html
That said, having the legal right to something, and being able to enforce it, isn't the same thing (just ask Adobe).
(On a more serious note, while I'm not a supporter of the draconian DRM schemes that have been enacted into law -- and while the studios make completely ridiculous claims for damages -- implying that films aren't being copied in a way that conflicts with their licensing terms is clearly wrong)
Although I don't think he should go GPL, he should at least know about the license differences.