When the developer does not understanding the GPL.

Unless he trademarked it, he can't do this. It's impossible to make it under a commercial license while being under the gpl, and even when trademarked, if we simply change the software's name, it's not trademarked anymore.

Honestly, this is pretty weird though, since we can use the GPL v3 license to bypass their "one-time purchase of a commercial license".

The problem is, if you tell the developer, he will probably put it under the CC BY-NC, or a similar nonfree license.

However, because it has been licensed in this manner, it can be downloaded and redistributed under the GPL, commercially and noncommercially.

A better way of making money would be a *strongly* suggested donation, or a paid download that is GPL licensed.

I've also seen this kind of misconception in a distribution of game art called the "SpriteLib GPL"[0] (now just called "SpriteLib", getting rid of the misnomer); it's under the Common Public License, which the author misinterpreted as meaning only non-commercial use, if I remember correctly. At the time, they didn't actually link to the text of the CPL or include it in the download, so I once e-mailed them asking for clarification, and they fixed that. Now, it seems they've removed their incorrect claim.

It's possible that this guy will re-license it under a non-free license, but I think it would be best to tell him that the GPL does not disallow commercial use while emphasizing that a license is only open source if it allows commercial use. (I suggest using the term "open source" because he might infer wrongly that what you're saying doesn't apply to open source, and ignore it as a result, if you only mention free software.)

[0] http://www.widgetworx.com/widgetworx/portfolio/spritelib.html

Is it possible that the developer meant proprietary instead of commercial?