Do you have external investment? Do you have any users?

How close is your friend?

Get a lawyer, and ask about the steps to get a friendly written agreement. At the beginning of a startup the standard agreement is a 4 years vesting with a 1 year cliff. If the initial split is 50%-50%, with the 4 year vesting your friends gets 3%, but with the 1 year cliff your friend gets 0%. I don't know how this work when you don't have anything in written and you want to split your ways now. Get a lawyer!

One of the popular lawyers here is https://news.ycombinator.com/user?id=grellas , but I don't know if he does this type of work.