Lots of businesspeople manage low-equity (or no-equity) employees. Their business isn't jeopardized when one of them threatens to quit. Some of them are savvy enough to know how to keep projects small and focused and checkpointable, so they're never more than a milestone into the weeds and can always hand working code off to the next developer. Others simply pay more than 80k for a lead developer. Many of them pay large premiums to hired gun consulting teams who will deliver pro work without caring about equity.
80k may sound really expensive to you. For a startup, it kind of is. But it's right smack in the middle of the ballpark for an experience Python webdev. It doesn't sound like you were really doing this person a favor. Chances are, when she poked her head up and looked around a few months into the project, it seemed like she was doing all the work and taking a sucker's cut.
She doesn't have to be right about that. She gets to do whatever she wants. People renegotiate comp all the time. You didn't sell her. She left. Learn more about managing people before you hire or contract more of them.