You have an deal to make with each other, and then some work/money to do/spend to make it legal. I would encourage you not to engage a lawyer until the two,of,y

You have an deal to make with each other, and then some work/money to do/spend to make it legal. I would encourage you not to engage a lawyer until the two,of,you have informally agreed, including via an exchange of emails, on the deal. You do not want to pay lawyers to negotiate the overarching deal. Understand what you need to have to continue. Would you walk away at 50/50? How about at 60/40? What’s the highest percentage share that you are willing to let your co-founder have? Understand how you value the business. Would you sell your share to your co-founder for $1? $10,000? $100,000? $1m? Similarly would you buy your co-founder our for any of those prices? Understand how it might work going forward - while the technical work may be done, the hard bit now is getting sales traction. Isn’t this when the other founder starts adding her/his value? And vice versa - the co-founder needs to understand all this too. And then have a coffee and work it out. Be generous to each other, and if there is no overlap then one person needs to buy out the other.