Should you replace the loss of the single most vital resource you have as a startup? I think so. Unless you've got the energy and motivation to make up for it yourself.
i have the confidence and motivation to go it alone, however, i think startups work better with twos. most of the initial work is done. but post launch a one man band may be a little too much!
Better with two if both are dedicated. Otherwise the second is a negative. You need a second to keep you motivated and moving when things on your plate slow down or get painful. A second that sits most of the process out is not part of that equation.
If you need their investment, negotiate based on it as an early investment rather than as a partnership (a partner gets lots of sweat equity and a pittance based on some small amount of cash invested--equity bought with cash is usually much smaller and on less favorable terms in the sense that small sums do not bring seats on the board, etc.). But you probably don't need that money. Buy back their share (if you've already incorporated and issued shares), and call it a friendship rather than a business partnership. You do not want to deal with these questions down the road when the company has assets.
