Went through this exact nightmare. Co-founder owned 50% with no vesting, contributed for 6 months, then ghosted while I built the product for 2 more years. When we finally got acquisition interest, he showed up wanting his full 50%.
Ended up settling for him getting 15% (from 50%) after expensive mediation. The mediator's logic: he contributed meaningfully during formation but abandoned the venture, so a reasonable equity split reflected actual contribution. But we had no written vesting — the only reason he accepted 15% was because he didn't want to spend $50K+ on litigation with an uncertain outcome.
Lesson: GET VESTING IN WRITING ON DAY ONE. It's an uncomfortable conversation that saves you from a devastating one later.