You failed your marketing/growth part. You have a minority stake if vested. Your cofounder doesn’t want to work with you anymore and starts something else. You

You failed your marketing/growth part. You have a minority stake if vested. Your cofounder doesn’t want to work with you anymore and starts something else. You had a free shot at your idea. It failed. Better luck next time. If you really think you’ll make it work, make the guy an offer for the company, and continue.