alt Hacker NewsShow it. Pick a missed edge case or breaking point in his app and demonstrate the pain the customer is going to encounter. You don't have to live in the realm of hypotheticals. He has given you a concrete, but flawed, implementation that offers proof of your message.
I did it and he "fixed" it with AI, making him even more convinced of it.
I'm pretty sure something else must have broken but didn't see it immediately.
I guess eventually it will fall from its own weight but I'm surprised how far he was able to get.