Cofounder Offer

Got an offer from a one man company. I think the idea is solid but that doesn't matter. It may not even work who knows. He has prior business ownership experience. I have 10 years of software development experience. He basically paid 100k to vibe code an app and paid some developers part time to glue it together. It looks pretty, but there's a lot of things that don't work. Only 1/5 essential features works and so far only on demo data. He offering me a s