It's not. He made an agreement, and bailed on it later, for no other reason than that he didn't like the agreement he had made. Doesn't matter what the numbers are.

Your analysis is too rigid and could use the nuance numbers might provide.

If the developer executing the idea was paid a low sum in exchange for low equity (say, 75K + 0.1%) and market analysis revealed a much smaller market than anticipated, the developer may have rationally understood the situation to be exploitative.

In such a case, the numbers might reveal that higher equity (say, 75K + 20%) might make the situation would not only be equitable but potentially profitable.

You're right to say that the developer breached the contract, sure.

However, having numbers might contextualize that breach of contract so that it could be read as a poorly-considered evasion of responsibility or a rational decision to leave a financially abusive and career-limiting contract.

EDIT: Readability. Add conjunctive adverb to last paragraph.

It's his life. People change, and he probably found something better elsewhere. Why would someone who's paid less than 0.1%(assuming the worst case) be forever loyal to a business? Loyalty has a price and it's much higher than most people seem to think.

I think it is a bit unfair to just listen to one side of the argument. Did you have a chance to talk to the developer to see if this is indeed what happened?

Agreements can be remade when circumstances change. Again, your business guy sounds pretty dumb when his solution is to learn it himself.

Depends if it was a "right to work" situation where no notice period was necessary - another crucial piece of the story omitted.

Sounds like the developer was just being a shrewd businessman.