I've had your experience, I lost both money and a friend. People tend to over-value their contribution. If things are already working well and you're making profit, then you need to quantify how and when this guy is actually going to add value any offer should discount such a projection with an incentive component to reward over-performance.

If you need emotional support, then either book a series of sessions with a shrink or get a puppy. Long-term personal success is a function of your emotional resilience. So wanting to get your friend involved at any cost is a warning sign. For some people a mentor might be a good alternative.