The developer was obviously brain-picked for any implementation ideas, as stated at scale. They should have been paying a retainer, or had an offer inside of two weeks.
Let this be a warning for other developers.
Some open source guy wants to pick your brain: Sure lets get lunch and split it.
Some small single digit founder start-up wants to talk abut your work: Ok sure, pay for my lunch lets talk about how I can help you change the world.
Freaking Microsoft wants to talk: That'll be 1k an hour plus expenses (also get a limo and a nice dinner).
The humility of engineering should stop at the boundary between people who want to change the world and those who just want to profit off of you.