We're basically talking about the same thing. But, yes, you are 100% correct about the likely danger in trying to use the code in the future. That's why I think the cleanest thing to do (if at all possible), if he wants to make something of this, is to offer the guy a small equity stake in whatever entity is created to do this work and get a release.

I've been involved with lawsuits over the exact type of situation. Maybe the OP could win, but it will be a costly process...