We had a similar situation with a co-founder in a previous venture of mine. It just wasn't working out, as he was unable to make the jump to a full-time commitment and that's what his position would have required.

The entire separation process diverted a lot of time and resources away from the company and our pursuit of the mission. That said, we did end up working with an attorney to tie up the separation as neatly and cleanly as possible.

My suggestion would be to find a qualified attorney to draft the agreement. That way you won't be spending any additional time and energy trying to put the situation behind you, since you'll have made the investment to handle it the right way, the first time. Also, if there is any intellectual property (IP) involved, make sure the IP rights are assigned from the departing partner to the company.

Happy to discuss further if that would be helpful and good luck!

Chris