- Make it clear to the co-founder that the business will be between much more likely and 100% likely to shut down if they maintain this complaint. While this seems like it's showing them the power they have, it's much more important to show them how fragile the value of any shares they have are.
- Make an offer between the original offer and 100% vesting. Make it cash heavy if you can, possibly contingent on funding. Make this offer exploding - they need to decide within 1 week and if they don't, you will pull everything except what you feel legally required to put on the table (the vested stock only, no benefits, no severance) and fight this.
This will drag on and kill your company if you let it, and settling could work. If settling doesn't work, you're in a new position, but you need to get this in a position where it will be resolved or not resolved quickly. Then you will know what you're dealing with more clearly, but you are not left with the binary choice you think you are.