Hire a process server to deliver a letter to your cofounder. Be polite about - just say that it's important that the two of you talk. Either they should come

Hire a process server to deliver a letter to your cofounder. Be polite about - just say that it's important that the two of you talk. Either they should come back and work with you, or the two of you should part ways. Imagine what might happen if you _don't_ break up, and your company is a success. Your cofounder would sue you for their share.