Maybe you're thinking about it in the wrong way. If you were to commission somebody to create a painting, you wouldn't paint over his or her name, scribble your own over it, and try to take credit for the work, would you? Even if you bought an unsigned piece of artwork for investment, you wouldn't go around telling people you created it, would you?

Sometimes it all boils down to something as simple as respect or attribution. If this desktop app is the next big thing, the coder probably just wants to at the very least make sure that he gets some kind of acknowledgment for his work. For some, acknowledgment can be as simple as ensuring nobody rips off their code and reverse-engineers it, and stamps a different name on it. For others, it is about monetary compensation.

I don't know what this dude's deal is, but of all the factors in your little moneymaking scheme, having the ability to write working code is probably the most valuable. 10 percent doesn't seem like a very equitable exchange.