Not to blame either side, but this is one of the reasons Founder Vesting is a thing. There should be a mechanism in place which strips the founder of equity when they quit. A lot of venture firms these days require it. But even if you not plan on rising venture capital, it's a good thing to have in place just in case.

It's a sort of thing you'd set up before starting to work on the company, to know what the ground rules are from the very beginning. Knowing that your stake is not a given is very humbling.

You can always have things like accelerated vesting in the event of a liquidity event, if the company is sold prematurely.