Maybe try to remember the reasons you originally wanted to start a company with them? I'm curious what those reasons are if this person is incompetent and trying to start a company. Did they bring the idea AND the money?

I'm in a related situation. I've been working for a company for years, and the owner is incompetent in most areas except sales. His original company is over a decade old but now almost dead. Somehow he's still getting investor money and for the past 5 years has been trying to start a new business, but keeps pivoting before anything is actually released to the public. I'm the main dev and it's getting frustrating, but I know without me he definitely won't get anywhere. And the pay vs time invested is pretty good at the moment.